What would be the best price for a second-hand iPhone 5?

I'm a student living in Denmark and prices can be very steep here.

Usually, the cheapest SH and undamaged 16GB iPhone 5 you can find is priced at 530 dollars. From what I understand, the phones are not sim-locked, but is it worth it?

Other than E-bay, where else can I find some reliable sources to buy one? Or at least, a reliable E-bay seller?