Easy, find a forum
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Find the forum that matches what your selling. There is literally a forum for EVERYTHING on the interweb!
Keep in mind if it's say a Hot Wheels forum the HW people will assume that your selling dirt cheap, rock bottom prices sometimes, or any other forum. I just use the HW forum as an example. Something about folks assuming that because they are on a forum with other forum users everything will be super, super cheap. Hell some users might even try to interject their own ridiculously low price offers. Some times insultingly even...