i like to go through them and get everything 'better than new' condition mechanically but leave the old paint and patina.
a buddy of mine goes through them, repaints them with auto paint, sorts everything out and gets them running better than new, usually with a mild kit. He puts about 10-15 hours of work into each bike and sells them for 1200 or so, ready to hop on and ride. Its definitely not paying the bills but if you are good at the restoration work and have a system dialed in, this is about the only way you can make profit.
most people out there trying to do moped 'restorations' are hacks just polishing a turd and trying to flip it for ebay money. I say most because that is a lot of what i see, but i'm sure there are lots of folks like my buddy that don't make the front page.
any moped from the 70's unless its in perfect garage/basement condition (and even then) needs a full rebuild to be totally ready to hop on and ride. For me its a lot less work to start by tearing it all down and cleaning it up, fixing all the little stuff, and putting it back together- than it is to break down a bunch of times when all the little things go wrong then have to fix them one at a time.