it is very easy, just like buying from ebay.
Difference is that you have to pay transhipping fees to the person in Germany who handles the legalities of sending the fish to the uk. it can be very costly and recently i know someone paid nearly £200 in shipping and handling fees. another thing is that some of the sellers prefer to be paid via western union, which is easy to use.
IME it usually takes 3-4 days from purchase to receiving the fish.
Like ebay it is best to check the sellers ratings and always make sure they are above 98% positive ratings. sellers also use alot of aliases but never had a problem with that.