Kixify is just a site for random sellers like eBay. I sold an extra pair of Bred XIs I had on there with no problem. After it was sold, I printed up a shipping label with PP, msgd the buyer back with tracking & a thank you note. It all depends on who you buy from.
The site is still very new and is working out the kinks as they go along.
If I have another pair to sell I'm selling them on there. The reason why the prices are lower on there than eBay is because eBay charges $40-50 in seller fees, while kixify only charges like $20.
You can't make a broad claim on an entire site given one shaky experience, especially when in the end you were still protected by paypal.