Yep - echoing the above, unless it's something particular that is released, or it's a particularly good deal, then eBay is my source - even there some second hand prices are well up (and in many cases, well over priced!).
I tend to buy stuff (N gauge) off E Bay too. Not all of it is in perfect working order it has to be said, but the seller usually either repairs it or refunds so although it's a bit of hassle one is not out of pocket.
I think the price of second hand locos is ridiculous, I wonder if Covid had an effect (reduced production with increased demand ?), but also, are many new models of 1970s BR (e.g. class 40/45/46/55) even still made ? If so can't find them.
Best bargain I got was a Grafar Hall for £40 (incl P&P) which looked a bit scabby in the close up E Bay pics but looks fine on the layout. It has excellent traction and electrical pick up, and it hardly ever derails, even on my tight 1970s layout !
I am a bit flexible about coaching stock, I tend to buy Lima even though it is slightly out of gauge, partly because it's cheap but also I have found they run really well. I try to buy in multiple batches to save on the carriage (often hidden as "free")
I haven't had as much luck with waggons, quite a few seem to derail rather easily and/or decouple. One wonders if sellers get rid of ones which do that on their own layouts ?
Vehicles (and other accessories) I also try to buy in multiples to save on carriage, in fact one can usually buy this sort of stuff for less, particularly 2nd hand, at exhibitions as the sellers are not paying any postage and packing. I was in Rails the other month and got a bargain, they were just putting out a load of 2nd hand N gauge Oxford diecast*, no boxes but otherwise fine, so I bought ten of them at £4 each !
* I only buy Oxford diecast, I like the quality and the fact they're so sturdy, the opposite of 3D printed stuff, they are so weak it's untrue. I bought one on E Bay put it in a Jiffy then in my bag, but the roof had broken off when I got home !