Rob i used to be like you and care about bills of sale.
Can i assume you are a first time moped buyer?
If so take the advice that is given, forge one.
Secondly if you are really worried, call the DMV get them to run the VIN to see if it is stolen. Besides you should have gotten the VIN before hand and checked it if you were really worried.
I check mine all before hand to make sure.
I find that mopeds fall into a grey catagory were the rules of other vehicles don't apply. The whole bill of sale and title is one segment, where it is pretty easy to come up with one but not through the usual methods that people who drive cars would think of.
Just to let you know, the first moped i got i was as worried as you were and i spent a month tracking down the original owner (this ped had changed hands 4-5 times since then). All i should have done was forged a bill of sale and the that would have been the end of it. (but it was pretty cool tracking down the guy that owned the ped origally as a kid... he was a dentist now).