new 10g's are 10 bucks so about 2-5 bucks for a used one,
basically as long as NO reptile or small mammal was in the tank it should be fine, gotta be water glass though, its a bit thicker then terrarium glass.
make sure it holds water and no cracks, check the sealant corners for nasties and loose sealant.
I paid 200 for a 52g flat back hex with tank top light( nice light cost 140 bucks alont) and 3 filters 1 UGF
two aqua clear junk ones, ive since added my own filters.
even a quick smell of the tank will tell you if its ok or not.
to clean a used one just use bleach and a dechlor after the bleach wash. I use straight sodium theosulfate for my bleach cleaning dechlor but even novaqua will work.