tbh, a lot of the balancing issues come down to how the tyres are fitted. With Kahn's and Toyo's you really shouldn't be having too many problems but sometimes you do get a bad batch!!
For peoples information, there is a yellow dot on the tyre sidewall and this should be in line with the valve. By doing this, you will minimize the amount of weights needed on the wheel. A lot of places won't do this and therefore you end up with a lot more weights than are actually needed.
ebay is always a risk if you ask me...simply look at the other post on here about the Wii!! I'd always go with a reputable company, even if it does work out slightly more expensive. If you do buy off the net, you should ALWAYS check that the tyres are marked as approved for British roads otherwise in case of an accident, things could go sour very quickly as far as insurance goes!!