I think an inherent issues is always going to be the existence of chess engines.
Sure, some sites claim they have ways to detect these engines, and ban players. But then it comes down to how do you ever know for sure that the site is actually banning cheaters or just banning people who win too frequently, and ensuring someone on the inside wins?
You might find a place to play for BTC, as others have listed... but you can never know if it is going to be fair at all.
Indeed that is very serious issue to play against bot and I don't think there is any authentic method implemented to detect these engines, but I don't think that is good idea to play chess for btc because it take so long time to get complete a game there, I think better idea to play just for fun there because so many site offering this to play online with other players.