BUY IT. If it is in decent condition and has not been mangled in a crash, or the gas tank is not rusted out it is worth twice the price. What you have there is a Moby 50V, commonly called a Mobylette. While they were made by the millions back in the '60s and '70s, not to many decent ones still exist today. The Gurtner carbs are finicky, and so are the electrics, but they are a lot of fun to tinker with and ride. Parts are still available for them. I would gladly trade my 1977 Puch for a nice one.
Back in 1978, a guy named Walter Muma rode one of these 11,500 miles across Canada and Alaska.
These are French, so they do require a little more "owner involvement" than a German moped like the Puch, but they also have a charm that very few other mopeds have.
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped