Where would one be able to find a large selection of second hand mobile phones (Android based) in Chiang Mai?
Properly testing a used mobile phone takes quite a while, so, do these places give you some kind of warranty?
Thank you everybody!

The used mobile phone may test well, but the battery may be spent. Best to find one with a replaceable battery. I would also look for one that is factory and not rooted

North end of Santithem Rd... A block of phone shops on both sides of the street. They offer a one-month warranty.  I've bought two iPhones there, two years ago. No problems.

Excellent, i know where they are.
One month of warranty should quite be sufficient for testing.
Thank you very much everybody!

Any stores specializing in batteries for phones?

Once you settle on a brand/model look a the Facebook Marketplace... Many local sellers post there... You can compare many seller prices...