Be very careful when you buy. A lot of the cheap EBAY jobs are very small and underpowered. The real problem is the spares when anything goes wrong and the fact that most of the really cheap jobs are Chinese and have rubbish build quality. If you are only going to travel very short distances (under 2 or 3 miles) on very flat, quiet roads and you are perhaps under 11st then, ok.
You can get better models but you will pay a lot for them. Why not get a used moped instead?