eBay is an evil place as it is oiled by greed...
That said, you can get great deals if you look in the right place, search well and are patient. A bit like fishing in a canal.
Regarding searching, find out how the searches work and you'll get the most out of the system.
For example, put these into your eBay Searches:
Hornby French Tin-plate O gauge:
Lenz Digital stuff:
Lenz (Set-90, Set-100, LV102, LH100, LH90, XPA, LS100, LS150, LB101, BM1, BM2, BM3, LG100, LK100, LT100, LR101, LB050, LRC120, LV101, LV102, LV200, LC100)
Arnold Digital stuff:
+Arnold (86201,86210,86220,86205,81207,86250,7098,Schaltrelais) -Schoenberg
Marklin Traverser (still trying to get a good one under 100 euros):
+(Märklin, Marklin) +(7294, SCHIEBEBUEHNE, Schiebebühne, "Transfer Table", traverser)
These are a sample of the 20 or so searches that I have on the go all the time. Every day I get a bunch of emails showing me what has cropped up on the searches.