Manufacturers can stipulate restrictions on where their goods are sold. Many do so to protect the value of their brand. In this case it seems AVG think that the incredibly cheap prices that can be found on ebay may serve to "cheapen" their product.
I understand that some brands do it and make it a point of their T&C'S when you sign up for them but if they are for genral distribution (i.e i did not source from AVG direct but through a distributor who buys from them) then is it still imposable ???
Most of the "brands" not allowed from eBay you have to source direct from the brand for this to come into force.