Please don't discard. If you are worried about the chances of someone finding the book, you have zero chance if you discard it. There is always a chance if you release it in the wild. Certainly not all books get "caught" with a journal entry, only a small percentage, but many are picked up and enjoyed by others, even if they don't write home, which also makes me happy.
I leave many of my books in restaurants, coffee shops, and waiting rooms. Because I don't trust the weather, I don't leave as many outside. And if I do, I enclose it in a Release Bag. You could offer it to the man selling books, but first be sure he is okay with a book with a BookCrossing label in it. Some sellers and used book stores and charity shops are fine with them, some are not and throw the book out or rip off the label, so I'd check first.
I personally don't think BookCrossing would be much fun if every book that was wild released was automatically caught and journaled. The fun is in the anticipation, wondering if and when a book will find the right reader who is intrigued enough to respond. But that's just my opinion. And a BookCrosser needs patience as even books that are picked up sometimes take a while to be journaled. Check out the Site Watch Forum to see some examples of catches.
You could offer your book in the Random and Not-So-Random Acts of BookCrossing Kindness Forum if you can't bring yourself to leave it somewhere. Either for free, or as a trade with another BookCrosser. I don't know if there are any MeetUps in your area, but that is also a good way to meet and exchange books with other members. You could ask in the Conventions and Meetings Forum.
Welcome from Maine. Good luck with your releasing! I hope your book eventually finds the perfect reader.