Best way is to ask seller to measure the bottom of the boat from the outside of transom (across the back where the drain plug hole is). A 48" bottom is ok but most folk prefer 52 or 54" as the gunwall height, horsepower, people / cargo capacity and stability usually increases too. Another clue for jon boats is : river jons are more narrow than lake jons. Titles are sometimes a problem, make sure they gottem in hand.
Yup, craigslist if fulla scams but I've also got some good deals too. Ask questions, use yer noggin n gut.