how much is he asking? and what condition(s) are the peds in? that makes a difference. for a running moped, i wouldn't go much more than $250-300 (unless it's super super well maintained and you are very desperate for it). if in some state of disrepair, go under $200 (or as low as possible).
see if you can get both for $500 or less. try it. he might not have an easy time selling them. so try to get both. like i said ... take a friend. he/she will want the other one. or buy them both and then you can let friends ride one so you can ride together.
that will also help build enthusiasm and bring more mopeds to your town. i've never met anyone who didn't ride a moped and fall in love w/ them. seriously. they're wonderful! ;-)