If you were buying from a Store online, you would have to pay in advance before they ship the product, this is the same practice that a Mold seller is doing for you and how anything purchased online is carried out.
A seller can't trust that he can ship you something in advance and have trust that you will ship it back to him, it could get damaged in the mail. You may not have done the damaging, but how is the seller going to resell it if you don't like it and he gets it back with a huge crack in it?)
There were at one point a lot of reputable users who would sell things they didn't need, or picked up at yard sales for resell. If you pay with Paypal or Check to where there is a paper trail, and then get a Tracking number for your item, you should be in good shape buying online.