I had a regret sell at one point and just kindly asked the buyer to please let me know if he ever decided to sell it. About a year later, he did just that and I purchased it back. I wouldn't have asked him to sell it back though, that's kind of strange. I agree with others that you are under no obligation either way. If you want to be super nice, help the guy out. If it is a hassle and you really like the bass, give a sincere apology and tell him you will let him know if you ever decide to sell it.