It's buyer beware. Read the sellers "Love notes" section. Happy customers pretty much means you won't be scammed. Follow all posh rules, it's safer that you won't get scammed.
Do your research on that LV bag before you buy it. There are fakes on the site, most sellers are honest, but you do have the scammers.

Dont purchase anything, its a 1000% guarantee you will get ripped off, read this site and other consumer reports concerning this horrible scammer haven

I agree with Katie G.

Hi. I've purchased over 20 different things on posh and I have never had an issue. Although, I know that some people have had issues. First thing I would suggest, is know the info about what you want to purchase. If you're looking for a Louis Vuitton for example, know that the letters and numbers in the date tag have to be arranged a certain way for it to be authentic. Stuff like that, to make sure you don't get screwed. Also, look at other listing the seller you might purchase from has sold, it may reveal that the seller is a scammer, because if the buyer has a problem, most likely the first thing the buyer will do is comment on the listing. This has saved me from potential issues a few times. Just be careful. I've met several great people who I will purchase from again, and also some people who try to scam. Just do your research on the seller before you by from them.