I've bought many licences from this source and have never had a problem. All my PCs are running all the latest updates and pass WGA. All installed from one retail disk on licenses from TrustPrice (except for one that came with my retail disk.)
In fact, the install disk I use is a copy of my retail WinXP SP1 disk with SP2 slipstreamed into it.
I have access to XP disks purchased at various times, so I do all installs and reinstalls from the latest disk.
The installation is sensitive, however, to whether the disk is retail or OEM, install or update. I had one disk advertised as a retail disk that turned out to be a corporate license.