It is actually possible to do this with just 3 players.
Here's most of the solution. I've written this up months ago, and I don't want to finish it now, but here's what I already had written up:
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Kingdom: Stonemason (Sm), Black Market (Bm), Masquerade (Ma), Watchtower (Wt), Noble Brigand (Nb), Fortress (Fo), Procession (Pr), Catacombs (Ct), Haggler (Ha), Goons (Go), King's Court (Kc)
Ruins deck: 10 Ruined Market (Rm), 10 other ruins
Other Cards: Copper (C), Silver (S), Gold (G), Potion (P) Hovel (H), Necropolis (N), Overgrown Estate (O)
Black Market Deck: Young Witch, Baker, Hunting Grounds, Quarry, Expand, Bank, Council Room, Margrave, and more
P1 draws CCCCN
P2 draws CCCCC
P3 draws CCCCC
Turn 1
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Player 1:
play 4C,
Buy Nb1, P2 reveals HC, P3 reveals HO, gains Copper
Clean-up: draw CCCOH
Player 2:
Play 5C
Buy Sm1, overpay 3, gain Ma1,Ma2
Clean-up: draw CNO,(reshuffle),Ma1,Ma2
Player 3:
Play 5C, spend 1 coin token
Buy Sm2, overpay 4, gain Pr1,Pr2
Clean-up: draw CCN,(reshuffle),Pr1,Pr2
Turn 2
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Player 1:
play 3C
Buy Wt1
Clean-up: draw WtCCCC
Player 2:
Play N
Play Ma1, draw CC (P1 -> Wt1 -> P2 -> Ma2 -> P3 -> C -> P1. With this notation I mean: P1 passes Wt1, P2 passes Ma2 and P3 passes C), trash C
Play Wt1, draw 3C
Play 6C
Buy Pr3
Clean-up: draw Sm1,H,O, (reshuffle), Wt1, Pr3
Player 3:
Play Pr1-Pr2
- Play Ma2 twice, draw HO (P1 -> C -> P2 -> Wt1 -> P3 -> H -> P1), draw CC (P1 -> C -> P2 -> Pr3 -> P3 -> O -> P1), trash Ma2, gain Fo1, trash it with Watchtower, put it in hand
- Play Fo1 twice, draw CC, trash Fo1, put Fo1 in hand, gain Ca1, trash it with Watchtower, gain Pr4, put it on deck
- trash Pr2, gain Ca2, trash it with Watchtower, gain Pr5, put Pr5 on deck
Play Pr3-Fo1, draw Pr4,Pr5, trash Fo1, put Fo1 in hand, gain Ca3, trash it with Watchtower, gain Pr6, put it on deck
Play Pr4-Pr5
- Play Fo1 twice, draw Pr6Sm2, trash Fo1, put Fo1 in hand, gain Ca4, trash it with Watchtower, gain Pr7, put it on deck
- Play Pr6 twice:
- Play Fo1 twice, draw Pr7C, trash Fo1, put Fo1 in hand, gain Ca5, trash it with Watchtower, gain Pr8, put it on deck
- Play Pr7 twice:
- Play Fo1 twice, draw Pr8C, trash Fo1, put Fo1 in hand, gain Ca6, trash it with Watchtower, gain Pr9, put it on deck
- Play Pr8 twice:
- Play Sm2 twice, trashing Fo1 twice, gain Wt2,Wt3,Bm1,Bm2, trash Sm2, gain Bm3
- Play Fo1 twice, draw Pr9, (reshuffle), Bm1, trash Fo1, put Fo1 in hand, gain Ha1, put it on deck
- trash Pr8, gain Ha2, put it on deck
- trash Pr7, gain Ha3, put it on deck
- trash Pr6, gain Ha4, put it on deck
- trash Pr5, gain Ca7, trash it with Watchtower, gain Pr10, put it on deck
Play Pr9-Fo1, draw Pr10,Ha4, trash Fo1, put Fo1 in hand, gain Ha5, put it on deck
Play Pr10-Fo1, draw Ha5,Ha3, trash Fo1, put Fo1 in hand, gain Ha6, put it on deck
Play Fo1, draw Ha6
Play N, Ha3-6
(have 26 actions, $8, 1 buy)
Play Bm1, play 7C, buy Hunting Grounds, gain Ha7-10, put all on deck
Play Wt1, draw Ha7-10,Hunting Grounds,Ha2
Play Hunting Grounds, draw Ha1, Bm2-3, Wt2
Play Ha1-2,Ha7-10
Play Bm2, buy Quarry, gain Bm4-8, Wt4-7, Ma3
Play Bm3, buy Expand, gain Go1-10
Play Wt2, draw Wt3, (reshuffle), draw Go1-5
Play Go1-5
Play Wt3, draw Go6-10 and Wt4
Play Go6-10 (all subsequent buys give 10 VP)
Play Wt4, draw Quarry, Wt5, Bm4-6, Ma3
Play Bm4, play Quarry, buy Fairgrounds, gain Kc1-8,Fo2-3, put Kc's on deck and trash Fo's, putting them in hand.
(have 1 action, $33 and 11 buys)
Play Fo2-3, draw Kc1-2.
Play Kc1-Kc2
- Play Bm5 thrice, buy Bank, Council Room, Margrave, gain Kc9-10,8 Golds,Fo4-10,Bm9-10,Wt8-10,Sm3-10, topdeck Bm's, Council Room, Margrave, trash Fortresses, putting them in hand
- Ma3 thrice, draw Margrave, Council Room, Bm9-10, Kc3-4, 3 arbitary cards are passed around 3 times (ending up with the same player)
- Play Kc3 thrice
Next you want to buy 15 more things with the last 5 King's Courted Black Markets and gain 150 cards from Hagglers, draw your deck using Scrying Pool, Counting House and other draw cards, and then you can easily empty the supply in your buy phase.