(in progress)
Gaining cards from the trash is a power that has been long expected by many, and has been long derided by others. Many a fan card has attempted this, but never with the success that Graverobber has. Most of them have missed the point that the trash is for cards you don't want. In many games, trash-gainers would be stuck looking at Estates and Coppers, or worse, nothing at all. So a good trash-gaining card should include a way to fill the trash with cards that you do want. Which is what Graverobber does.
Well, what does it do? Graverobber offers you a choice: either Expand an Action, or gain something from the trash of a reasonable cost, putting it on top of your deck. All of this works together almost perfectly, and makes Graverobber a much better trash-gainer than even the other official one, Rogue. I've played several games where Hermits or Rats prevent Rogue from ever Attacking, and thus adding better stuff to the trash to gain, which is why Rogue needs the +$2 to compensate. Graverobber gets out of the trash exactly what you put into it, and not what you manage to pilfer from your opponent.
Graverobber, along with the Knights pile, expands upon the dynamic introduced by Tournament. Previously, the competition was to see who could get the most of a card, whether it be Cities, Wharves, Minions, or Provinces. Tournament added a mechanic where the competition is now for a specific card, of which there is only one copy. Many players dislike this aspect, but I personally find it diverting and enjoyable. Knights and Graverobber expand this concept by removing the "race for more of a pile" entirely. Instead of an intermediary Province race, the race for the one specific card is immediately on. So if there's one card in the trash you really want, you'd better grab it before your opponent does.
Perhaps the best Graverobber strategy is to trash $5 Action cards to gain Provinces. With no other $5 on the board, Graverobber itself can fill that role, but Graverobbing Graverobbers into Provinces tends to clock in at about the same as pure Big Money, as there will be a couple turns where Graverobber does absolutely nothing if you buy it too early. So this strategy works best either within the context of engines, where you're buying lots of Actions anyway, or with Action-gaining cards, such as University. What differentiates Graverobber from other trash-to-replace cards is that Graverobber can put trashed cards back on top of your deck (as long as your opponent doesn't take it first).
When you're using Graverobber to pick up the leavings of your opponent, chances are there was a better $5 card on the board which your opponent went for, which you should have gone for. If your opponent Developed a Tournament into a Menagerie and a Mountebank, the correct play is not to Graverob the Tournament. The correct play is to have bought the Mountebank in the first place. Graverobber is a good card, but it is not a board-dominating card. It is a supplemental card, except in the absence of junking Attacks, when it becomes a strategy on its own, as previously described. Now, if you do find yourself with Graverobber in hand, and something tasty in the trash, snap that up before your opponent does.
How can I help my Graverobber? While Graverobbing Graverobbers can work, this can be made faster by filling your deck with expensive Actions, particularly within the context of an engine, as mentioned previously. If they're non-terminal, even better. Cards like Market, Bazaar and Junk Dealer are great for this. Grand Market does well with Graverobber because you can Expand a cheaper Action into it, forgoing the need to rid yourself of Coppers. That Grand Market can then be turned into a Province if necessary. Border Village is awesome with Graverobber, because in buying one, Expanding into one, and gaining one from the trash, you acquire an extra card. And Border Village games tend to produce a superfluous amount of Actions, so you don't feel so bad turning those Border Villages into Provinces.
You also want to maximize the chance of pairing Graverobber with a relevant Action even further, by either trashing away non-Actions (making Graverobber combo rather well with other trashers, particularly non-terminal ones like Spice Merchant and Junk Dealer), or by high draw and/or sifting (again, preferably non-terminal, such as Laboratory or Warehouse). Laboratory and Junk Dealer are, in turn, good targets for Expansion into Provinces when the time comes.
When should I stay away from Graverobber? Graverobber's power is much diminished in Colony games. It will only ever be able to Expand a Peddler into a Colony (barring lots of Highways), and while Colony games tend to have $6 and $7 cost cards, you should be more concerned about stocking up on those, than on getting a Graverobber to turn them into Platinums. This, of course, is if you're actually going for Colonies and not trying to rush the Provinces. But if your opponent gets just one Colony, he can still win even if you split the Provinces 5-3, so be careful.
How do I counter Graverobber? Buy the more powerful card, or, if Graverobber is the prime $5 on the board, mirror a Graverobbing Graverobbers strategy. For Attacks, the best are Looters and Cursers. Ruins are too cheap for Graverobber to usually do anything useful with them, except getting a Silver, and it has no idea what to do with Curses. Discard Attacks typically will not hurt a GR/GR strategy. You may be tempted to Pillage to knock one of those Graverobbers out of their hand, but the Graverobber-er will simply put that freshly trashed Pillage on top of his deck and use it on you next turn. Deck inspection is usually useless here, and Graverobber can gain back cards lost to trashing Attacks, though this does slow your opponent down.
Now let's mention a few cards that Graverobber interacts with especially well.
Fortress changes Graverobber from a replacing card to a gaining card (so long as the gained card is less than $8). Graverobbing Fortresses will never get you a Province, but it can get you Golds and King's Courts, and lots of them.
Be careful with cost reduction around Graverobber - just a little, and you can gain Provinces from the trash; too much, and you can't gain anything from the trash. This is convoluted, but if you can make it work, it pays off in droves: reduce cost so that Provinces cost $6 or less, then with either Saboteur or Knights, trash an opponent's Province (cards like Spy are great for ensuring you hit a Province, though cost reduction raises the chance anyway), then use Graverobber to gain it from the trash. This is one instance where Rogue is better, because Rogue does the deck-trashing AND the trash-gaining, so long as there's nothing able to be gained in the trash (again, cost reduction helps here).
Cards that trash themselves are simply begging for Graverobber to come along. Excepting Embargo, which is too cheap, every self-trasher can be brought back to life with Graverobber, and be used on your next turn. Madman strategies no longer have to be insanely fast, and Pillaging is no longer limited to ten strikes. Graverobber even allows you to forgo buying these cards in the first place. Just wait for your opponent to use them, and then snatch them up on your turn.
Since Graverobber likes to trash $5 cost Actions into Provinces, cards that give you $5 cost Actions do well with it, and in general any card that gives you extra cards is of benefit - Altar, Border Village, University, Haggler, even Catacombs.
Does well with:
Fortress
Self-trashers
Border Village
Counter with:
Cursers/Looters
More powerful cards
