Hamfests are probably the best place to find used radios, but you must be familiar with the equipment. Hamfests are repleat with older radios you won't see in today's catalogs. At last count, there were over 140 scanners and monitor receivers of various brands in my collection. I purchase most of my receivers at hamfests or horsetrade with other radio hobbyists. Buying a used portable scanner is riskier than buying a mobile or base model.
Portable scanners are subject to more physical abuse and many have been dropped.
If a radio has been dropped, the laws of probability dictate that the first point of impact was probably a corner, so be sure to examine each corner for evidence of trauma. Scanners used in mobile service are subject to vibration, dust, and temperature extremes. This shouldn't dissuade you from buying a used mobile scanner, but be aware of possible complications. You should have some recourse if the radio you buy turns out to be defective. If you can't fix the radio yourself, you can pay to have the manufacturer or a service clinic repair it for you.