In general, do people feel obligated to respond to every unsolicited email they receive? I don't. If I were to do this, it would simply be too time consuming and I don't wish to spend my time writing back to strangers who work for Nigerian banks, are trapped overseas with no money, etc.

I also feel that if offering free stuff here means that you must/should respond personally to every expression of interest that your offer generates, it would discourage some people from attempting to gift this stuff to others as the process becomes just too much work for the one who's trying to be generous. Most folks tend to repost in the original thread when all of the goodies have been claimed a statement that everything is gone, sometimes as a new post and sometimes as an edit added to the initial post. IMO this is perfectly adequate.