So, I guess people want me to post my expert mode solution - I'm sorry I didn't do this sooner, but f.ds was down for like a whole day. Anyway...
Kingdom includes: Lighthouse (if non-solo), Ambassador, Villa, City, Rogue, King's Court
Events: Bonfire, Ferry
Setup: -$2 token on King's Court, all opponents defended, 2 piles empty
Hand: 2xKing's Court, 2xRogue, City, Villa*
Deck/discard: empty
Trash includes: 4xKing's Court, 4xRogue, 3xCity, Ambassador
*(The Villa is not in your hand at the very start of the turn, but it is there at the start of each subsequent cycle.)
ACTION PHASE
Play King's Court
-> Play King's Court
->> Play Rogue, gain King's Court (+$2)
->> Replay Rogue, gain Rogue (+$2)
->> Replay Rogue, gain City (+$2)
-> Replay King's Court
->> Play City, draw King's Court, Rogue (+1 buy, +$1)
->> Replay City, draw City (+1 buy, +$1)
->> Replay City, draw nothing (+1 buy, +$1)
-> Replay King's Court
->> Play King's Court
->>> Play Rogue, gain King's Court (+$2)
->>> Replay Rogue, gain Rogue (+$2)
->>> Replay Rogue, gain City (+$2)
->> Replay King's Court
->>> Play City, draw King's Court, Rogue (+1 buy, +$1)
->>> Replay City, draw City (+1 buy, +$1)
->>> Replay City, draw nothing (+1 buy, +$1)
->> Replay King's Court
->>> Play King's Court
->>>> Play Rogue, gain King's Court (+$2)
->>>> Replay Rogue, gain King's Court (+$2)
->>>> Replay Rogue, gain Rogue (+$2)
->>> Replay King's Court
->>>> Play Rogue, gain Rogue (+$2)
->>>> Replay Rogue, gain City (+$2)
->>>> Replay Rogue, gain Ambassador (+$2)
->>> Replay King's Court
->>>> Play City, draw 2xKing's Court (+1 buy, +$1)
->>>> Replay City, draw 2xRogue (+1 buy, +$1)
->>>> Replay City, draw City, Ambassador (+1 buy, +$1)
Play Ambassador, return Villa*
BUY PHASE
$33, 10 buys
Buy Bonfire, trash 2xKing's Court (-1 buy, -$3)
Buy Bonfire, trash 2xKing's Court (-1 buy, -$3)
Buy Bonfire, trash 2xRogue (-1 buy, -$3)
Buy Bonfire, trash 2xRogue (-1 buy, -$3)
Buy Bonfire, trash 2xCity (-1 buy, -$3)
Buy Bonfire, trash City, Ambassador (-1 buy, -$3)
Buy Villa, put into hand, return to action phase, restart cycle (-1 buy, -$4)
TOTAL ACCUMULATION PER CYCLE: $12, 3 buys
I do realize this is messy as all hell. There was a version with only 2 King's Courts that was MUCH cleaner but it didn't accumulate buys (this was when I wanted to do an infinite turn challenge rather than an infinite coins/buys one). L3 Cities are perfect for this - you rack up coins and buys, draw everything you need, and the nonterminality means you don't need to use a KC slot on Ambassador. Do you guys think that's cheating though?
Here's an open-ended challenge for anyone who wants to try: Can you make this work without KC? I got REALLY close but there were roadblocks I couldn't get past without tripling. No need to satisfy any hard modes - just the main goal of (theoretically) infinite buying power.