Second hand smartphone

I would like to get an ublocked smartphone that I can use with my current Proximus subscription. That subscription is paid by work so I don't want to change provider, just the phone as the one I've got from work is not very good. I do not feel like paying the full price for a new smartphone at the Proximus shop and there are no deals for subscribers. Plus their selection seems to be very limited.

Any suggestions on how I can get a good deal on a good condition used smartphone that would be ubblocked and allow me to use my current Proximus SIM card? Ebay? Are there dealers somewhere in town? Any pitfalls to avoid? Or should I just get something from the US or UK?