1) Could be stolen.
2) Could be a scam and you'll get nothing at all; that happens on occasion even though eBay claims to provide protection against such issues.
3) Could be locked to a carrier and hacked to unlock it, which will void all support and probably would be relocked at the next OS update
4) Any "refurbished" iPhone is done by some unknown entity - only Apple can sell factory-refurbished products and they don't sell refurbished iPhones - so what was done during this "refurbishment" is completely unknown.
5) There is no guarantee that the iPhone was originally sold in your country, and an iPhone that came from elsewhere will get no support from Apple except in that country (if any would be possible after a "refurbishment" at all); you'd have to find the country yourself and send it there should any service be necessary.
It's really a risk to buy an iPhone (or any smartphone) from an unknown and unauthorized source. I strongly suggest you either buy from someone you know, or buy from an authorized iPhone outlet.