You won't get a fully-legitimate copy of XP from a retailer anymore, certainly not from the usual trusted channels (MS, Amazon, ebuyer, etc etc). Having said that, installing OEM copies (like the ones being sold by the German website) bend/break Microsoft's licensing rules only in a small way. Whether that's OK, is for you to decide.
OEM copies are designed to be used by people or companies who are building PCs. AFAIK, if you're not building a new PC and you install an OEM copy then you're technically breaking the license agreement. The license should validate OK though as long as it hasn't already been used, and it should be fine to connect to the MS website for updates etc.
I bought 2 copies of XP from ebay a while ago: they were marketed in a very similar way to those in the Amazon link, but they turned out to be duff. They were volume license copies which wouldn't validate on my PC. However if neilp9876 has used the German ones and they've been OK, then that counts for a lot. They're probably your best bet.
Any copy of XP sold by a retailer for less than £100 ish is almost certainly either OEM, Volume License or some other restricted-use license. A legitimate "full retail" copy of XP (the pukka version) is very hard to get hold of.