With eBay, you do have to keep an eye out for decent prices.
Sometimes stuff just pops up and if you don't grab it immediately, it will be gone within a few minutes. I've had that happen multiple times. Still kicking myself for not buying a lot with a Cyrix 5x86 133 in it.
Sometimes stuff pops up for a really good BIN price and I have been able to jump on multiple things like that.... when those same things are normally being listed for many times more than what I was able to grab it for.
Prices for a lot of retro and vintage hardware have skyrocketed on eBay over the past 2-3 years so it is getting harder to find good deals, but they are still there if you are patient.
If you are looking for exact brands / models then you basically have to pay what people are asking if it really means that much to you. If you don't care so much about the brand, then it gets to be much easier to find what you want for cheaper.