The credibility of the seller is absolutely unknown if they're new and no one ever met them before. I know many cases fellow members/my friends went over to buy some dry goods and end up catching cracks and leaks or broken parts where it is not described in the ad. If buyer was to come back and post what he saw on the forum about the item then I think the next person who is interested would appreciate this information because it just saved him/her some money and time.
Just my $40 worth of opinion.
This is a forum dedicated to help it's members. I bought my share of nonworking equiptment from people because no one told me better, I think we should save other noobs from making the same mistake. If I see a crappy product that does not work I will PM the prospective buyer and tell them some info and direct them to credible sites explaining why it does not work. Like in the case of a aquamedic denitricator.
Same thing about livestock. I've drove 65 miles to someones house who said he have ...... colored coral that is ............. inches and end up being totally false. Reef1973 will confirm this also. I ended up buying some of his stuff because I didn't wanted to be rude, despite already having 2934839 of them in my tank thats the same color. Reef1973 ended up doing the same thing, buying a frag to not be rude. So if I would have posted a comment about the coral, I know Rit would have saved some time, money and gas.