If this place has already screwed you once I would not recommend you give them another $300 so they can sell you a more expensive lemmon.
Quit while you are ahead.
Small independent used lots on the side of the road are the kinds of places where you are most likely to get screwed.
I would recommend you try to get your money back (good luck) and buy a better car from an individual through the auto trader.
My piece of shit acura I sold for $700 a little over a year ago sounds like it was in much better shape than what you bought, and it was a 1986.