I read the eBay auction and have this opinion.
If the bike is in the condition stated it would take minimal effort to get the bike running and keep it running. As far as the price, what is reasonable, goes that is a value judgment based upon what is avalible in your area, and how badly you want a moped. But as a guess based upon what I have seen in Ohio about $300.00 would be the turning point between a good deal and paying too much. Less than $300.00 would be a bargain and over $300.00 would not be a bargain. But, paying over $300.00 may be worth it to you if you really, really, really want the bike, and can afford it.
Also, paying over the $300.00 may be worth it to you to actually have a bike now. Ask yourself this question, "would paying $X.xx over a reasonable price, and having a bike now give me more satisfaction than paying less money 4 or 5 months from now for a bike?"
Of course if you get the bike you do realize that no matter how much or how little you pay you will find another, cheeper, and in better condition bike about two months after you buy this one. But this will only happen if you buy this bike.