You can try to find out or estimate how many copies there were. If there are tens of thousands, it may make you feel less bad about tossing them. If people want them, they can find them and it's not necessarily your responsibility. For many books the world's need for them really diminished in relation to what has been mass-printed at one time: popular fiction like airport books, self-help type etc.

Second hand bookstores often buy. You can offer your books to them. Some they won't want, but might accept as a donation along with those that have monetary value to them. In reality, they have similar constraints of space and expected time to when someone will purchase (which could be years for even semi-known books). People have this romantic view about books, but if you want to hold any kind of personal library and don't have a freaking villa, you have to learn to be pragmatic. Even if you don't have a lot of books, get ones that you would read.