There can be many reasons why a small business would have large variations year on year. Some are reasonable, some are not.
The main thing to evaluate is the tax returns. "If it's not on the tax returns, then it didn't happen." is the best rule of thumb to go by.
If it's not a profitable business, then you probably don't want to buy it. If it is profitable, then you have to dig down and find out why they're really selling. After all, who would want to sell a profitable business that practically runs itself?
If you provide more detail, we can provide more meaningful feedback.