That's usually the way it goes, every time I come on here and say my moped is running awesome, I'll go outside and the carb float will be stuck or something. lol
Anyhow, I think $300 is a fair price for it, but how will you test how it runs without the carb needle? If you can't test the mechanics of it, I would only offer $200. Not saying it's not worth more than that, but I think that's fair if you can't see how it runs.
I bought my Sachs for $300. I had planned on offering even less, but the thing is in really nice shape and I really wanted it. It didn't run, but I knew it was just something stupid that I could figure out because it wasn't seized, and it was.