The token gives you an action when you play a card from that pile, and your Estates are not from that pile.
Yup, but if you're +1 Action token was on the estate pile...
That pile ain't yours, son!
True. While Donald allready explained that starting Estates come from the Estate pile (so they are "cards from that pile"), Estates in the supply are not Action cards, and you can not put the Lost Arts token on them.
Without the once per turn clause, possession +borrow is broken
Why? The if clause still prevents you from using it more than once that turn.
So what's the point of the "once per turn" clause here? Buying Borrow more than once per turn would make no sense anyway, since there's no Treasure cards that "draw" cards (and thus remove the -1 Card token) AFAICS. Is it just for clarity, or due to some unrevealed Adventures cards?
You could:
1. King's Court all your Market Squares to get immmense amounts of buys
2. Play five Highways
3. Buy Borrow, getting +$1
4. Buy Cultist for $0
5. Trash it with Watchtower, drawing 3-1 cards
6. Repeat steps 3. to 5., getting +$1 and +2 cards per buy until either the Cultist pile or your buys run out
7. Spend your last buy on Colony, or buy some Farmlands to trash something you drew, or whatever you plan to do with a bunch of cards and/or +$1s
The same works with Rats, just here the pile doesn't run out as fast and you don't draw as much. Obviously it's still rather constructed, so there might just as well be some Adventures cards behind this.