Agree, OP, that people can be rude, but you control the process to a large degree. I really see my buy nothing groups as doing a favor for me. If someone doesn't show up, then you contact the next person as soon as it is convenient for you. People who are rude, you just ignore. You don't have to give your things to rude people.
For me, actually, the worst part of the buy nothing groups is when a known hoarder asks for my things. I ignore the request, but I do feel bad for the hoarder's family.