Originally posted by SelrahCharleS someone who specializes in selling cameras is probably going to be a more consistently good experience than something like Ebay or pawn shops.
Plenty of sellers on ebay are brick and mortar camera shops using that platform to do their online sales (or a portion thereof). KEH, Roberts/UPP, and Adorama all have used stuff listed on ebay. Many of the Japanese sellers with tons of great condition vintage gear are established camera stores in Japan.
There are of course individuals selling on there too; some small resellers and some the hobbyist or his/her estate. Usually you can suss all of that out while doing a little diligence right there within ebay.
Not trying to be argumentative or anything. I agree that a camera shop is the best place to buy a camera! Just don't neglect that some of them sell through ebay.
Also chiming back in about it because there's more than 100 Pentax MXes listed on there right now with various conditions and prices. Even if OP doesn't want to buy there it would be a good place to get a grip on pricing. Checking the recently sold listings is very informative there.
Hope OP finds a good camera.