I tihink the reason these have been selling so cheap lately is because the Matrix II is due to come out this summer.

By the looks of the pictures in the ebaylink, the Matrix you bought looks like it has the NX-26 engine. If you are not going to replace the engine I would recommend sealing the engine with automotive silicon around the carb and backplate. Also I would remove the needles and check the o rings. If the are ok then put a little plumbers tape around the threads and reinstall both needles.

The Cen Matrix is a really good buggy. It is gaining in popularity beacuse it is very durable and can be very competitive with the right engine and electronics. Good luck and I hope you enjoy your Matrix as much as I have mine.