Here are all of the +Buy cards split into categories: Terminal, Cantrip/Treasure, Village, Special
Terminal -- 15
Woodcutter
Council Room
Baron
Bridge
Salvager
Wharf
Herbalist
Trade Route
Goons
Horse Traders
Nomad Camp
Spice Merchant
Margrave
Forager
Ruined Market
Cantrip/Treasure -- 6
Market
Pawn
Contraband
Grand Market
Counterfeit
Market Square
Village -- 4
Festival
Worker's Village
City (activated)
Hamlet
Special
Squire -- gives 2 Buys, definitely include this
Tactician -- too much trouble to set up multiples; only use from BM
Princess -- only get this from Tournament out of BM
Sir Martin -- gives 2 Buys, only worth getting out of BM
Not +Buy cards that are relevant
There are enough +Buy cards that won't fit in a kingdom to justify Black Market.
King's Court is definitely wanted, and it is most effectively used on Squire.
Throne Room and Procession might be able to eke out some extra Buys if combined with KC... I don't want to work out the chain math though!
Band of Misfits can duplicate Squire to get a whole bunch more Buys.
General Strategy
The Kingdom should include Black Market. Grab all the extra +Buy cards that aren't put into the Kingdom. Use Tournament out of BM to get Princess. Grab Sir Martin and Tactician from the BM as well.
Your megaturn will begin after playing Tactician, to get that one extra Buy. For easier prep, you'll also have 9, maybe 10 Wharves played too, just so you can draw as much as possible at the start of your final turn.
You'll want Bridge in the kingdom, and you'll want to have a bunch of gainers from the Black Market (Ironworks, Workshop, Feast, even things like Remodel will work).
Start off with a bunch of the +1 Buy Villages. WV and City are the main ones to use, since they also have draw.
At some point, you'll play out a gigantic KC-TR-Procession chain (not sure how to optimize this) and triplicate 10 BoM-as-Squire, 9 actual Squires (need to leave one in supply for BoM) and Sir Martin. If any KC remains (again, I don't want to work out the chain) you can use them on anything, it doesn't really matter.
At some point you will play all your Bridges, then use gainers to grab the remaining cards in the Supply that you left there so as not to 3-pile. And then you'll draw them, probably with a couple cards out of the BM because the Kingdom has more important things to hold. You might do part of this at the start (to get the last KC/TR/Procession). You'll definitely do it at least one after the big KC chain to get the last Squire, after you've played all the BoM-as-Squire.
Then you play out all the other +Buy cards you have from the Supply and from BM. Remember you can also have 10 Ruined Markets.
Assuming you don't have to be able to buy anything, you'll want both Counterfeit and Contraband in the Kingdom, not out of the BM. This is because Counterfeit itself provides +1 Buy, which means Counterfeit+Contraband together provide +3 Buy. And actually, you should play Counterfeits on other Counterfeits to start, chaining them down to Contraband. That should provide you 39 buys (1 from the very first Counterfeit, 2 from each subsequent Counterfeit, because they are all doubled from the previous, 2 from each Contraband).
Definitely want in the Kingdom:
Black market
King's Court
Band of Misfits
Squire
Contraband
Counterfeit
Probably want in the Kingdom:
Throne Room
Procession
City
Worker's Village
Wharf
This puts us at 11, so something would have to be dropped:
- TR or Procession if they actually don't help the KC chain pump out more Buys
- Worker's Village if the BM can provide enough +action
- Wharf if the BM can provide enough draw
Want from the Black Market:
Tactician
Sir Martin
Tournament -> Princess
Gainers
Lots of Villages
Enough draw, especially non-terminal (Lab, Stables)
NoMoreFun's suggestion of Mandarin (to replay Contrabands) is interesting, but I don't think it is worth the Kingdom slots. You would need not only Mandarin but also Horn of Plenty in the Kingdom, so that you can gain Mandarin and still buy things. And then you'll also need Cultist in the Kingdom so that you can gain that with HoP so that you can trash it with Watchtower (from BM) and draw the treasures you just topdecked... no, it's not worth three Kingdom slots.