Co limps, I raise on the button with 8c8s, BB calls, CO calls.

BB..Unknown but has the look of a gambling type. This is his 5th hand and he has played everyone and has raised pre flop twice.

9s6d4d...Checked to me and I bet, BB calls, CO folds.

9s6d4d7s...He checks, I bet, He raises, I call

9s6d4d7sQc...He checks.......Is this too thin of a bet?

I discount him having a 9 a little because he should be motivated to raise the flop with the third player in the hand. I also don't see typical players delay raise with hands like this. I think a seven makes sense, Maybe he CR a picked up nut flush draw and gave up on the river but will now call with Ace high. Maybe he is being tricky and going for the sexy. Maybe he the Q scared him.

I would check behind, seeing a classic "what worse hand will call?" situation.

1. His range is wide as a "gambling type" vs. pre-flop action.
2. His "legit" range includes hands like A5ss, which would CR turn and CF river.
3. Because he is gambler, I think you are obliged to plan Bet-call if he CRs.

I love thin river value bets on principle, but this does seem too thin, IMO.

I like a river check back and I think you could check back some 9's here also. What about a check back on the flop?

I'm no flop check back expert but I would think this hand is too strong especially with a third player in the hand.

I don't like checking back the flop with your hand.

I would bet the river when checked to. I don't think you're printing some huge portion of a bet by making it but in my mind it's worth some small fraction of a bet.

Yes you should bet. I dont think thin value is the best way to describe your hand, this has to be close to the best hand you could have and still be considered thin value. Thin value might include going for value with a hand like ak, aj, and weak pocket pairs for example.

This is a classic "any pair and ace high will call" situation

that would be a disaster. 88 is somewhere between thin value and a home run valuebet depending on what kind of "gambling type" villain is, so any 9 is a massive home run for the same reason plus you're blocking 9s