Well this is a very big topic but from what I have seen lately : when random people with 2 posts say they are selling 20m PoE, get a few vouches from other random people with 2 posts, it is very likely it is the same person vouching for himself in order to get 25-100usd off you while getting you to go first.
I am not pointing fingers at everyone I am just saying, if the vouches do not seem solid I would not advise you to go first , maybe look for someone that is trusted in this site to miidleman.