I started to sell on Posh this winter. Since then I sold 70 items there (all high end), had only 2 returns and they were quite fair. Also once they refused to buyer and took my side when buyer requested return. There are many fakes and scammers, but my personal overall experience is great!
I guess selling would be fine but buying can be risky. A few years ago I got quite a few great deals there though. I guess they're no worse than eBay.