eBay is fine, just make sure the Air Law book is as up to date as possible. With EASA taking over from the CAA, there are loads of changes at the moment. Also, I don't know how up to date the CAA exams are in this respect. But a three-year old Air Law book is worth less than the paper it's printed on.
I also know that some Trevor Thom 7 (R/T) books which are still being sold are based on the old ATSOCAS and not the current ones. I got mine when the system had only just changed (a good 4 years ago I think), so it was understandable then but be careful now. As a student it's pretty confusing if you're learning the wrong services!
I'm assuming (so am probably wrong) that a new edition has been published (FlightStore says 2011), but it doesn't seem to be circulating yet from certain places. If I were doing it again I wouldn't bother with the R/T book anyway, I'd just download CAP 413 and the GA supplement.