Aquifer is a Peddler with a drawback (being that it's only non-terminal for other Aquifers) that comes with an extra benefit of a rarely relevant Reaction--especially since it can only be triggered during your turn which strongly limits the number of cards that can activate it.
Peddler is typically valued at about $4.5, and I think the drawback is significant enough to pull it down to $3-ish. That Reaction is not worth much. I'd guess Aquifer is weak.
Artifact gives a lot of really hard to math options since you have to think about the cost of the Treasure, the $ production of the Treasure, and that Artifact itself produces $1.
It's options come out to be: $1; or $2, gain a Copper; or if $>=$2 then $0, gain a Silver; or if $>=$5 -$2 gain a Gold; and I won't bother with what gaining additional Artifacts will look like since it puts you back in the loop. I don't think it's all that strong either, so I'm not sure it's worth the headache when you have the much more compelling Treasure Trove and Lucky Coin
Riches forced trash will typically be a huge benefit. Even just using it once with a bunch of Coppers to slim your deck down and never playing it again (as though it were Chapel) will probably make it really good, let alone that one could possibly play it with one or two Coppers to buy whatever you wanted and slim at the same time. It should probably cost $5, maybe even $6.
Whatever it's worth: I cannot imagine a Kingdom where I wouldn't open with Riches as written. Even if I got a 5\2 split, I'd probably only open favor strong junkers over Riches.
I would guess that Sarcophagus is overpowered. Multiplayer games will likely be miserably short with it between draining Action piles via Sarcophagi plays which will be common because of the on-buy that encourages Sarcophagi to be spam-bought. That on-buy is such a good effect that it takes a sub-$2 card into a $5 card in Ill-Gotten Gains.
I'd drop the on-buy and increase its cost to $7.
Collector, Grave Watcher, and Tomb Raider are cute and unique.
I like that Collector's benefit to all players works to enable other players' Collectors, while its one-card trashing is balanced by forcing to gain a card sharing a type with it (meaning that hitting an Estate turn 3\4 isn't a game-win). Could you name it Financier with that art? It might be a bit anachronistic.
Grave Watcher is a Copper junker, but is bounded by aligning it with a Curse which I think keeps it sufficiently in check.
Tomb Raider is probably more useful as a defense against other Attacks than a Treasure copier, but all its pieces come together nicely.