☰
[Editor’s Note: This answer is written by blackjack and online-gambling expert Arnold Snyder.]
I’ve never heard of a slot player selling his bonus rounds to another player and I suspect this situation is so unusual, it's unlikely that it would violate any house rules or gaming regulations.
I have seen blackjack players "sell" their hands to other players and I’ve never seen a dealer or pit boss disallow it. (Not that this is common, but I’ve played so much blackjack that I’ve witnessed just about everything that can happen at a table.) I’ve also observed many players make side bets with each other on hands at both blackjack and poker. I’ve never seen a casino disallow this type of gambling.
The only "legal implications" I can imagine in the slot situation you describe would be tax consequences, and those would only occur if the ticket cashed out is for more than $1,200, in which case the casino must get your Social Security number and fill out a W2-G to report the amount of the win to the IRS. In many cases, the casino will also withhold 25% of the win for the IRS.
If the way you "buy" the bonus spins is by paying the seller the current cashout value, plus an amount added for the bonus spins remaining, you could be stuck for tax consequences on the full amount cashed out if it’s $1,200+, even if a large portion of that went into the pocket of the player you paid out of your pocket.