I started buying razors on ebay a couple months ago and have been very happy with the results. I also need to STOP.
First, I bought "Standard Guide to Razors" by Roy Ritchie & Ron Stewart. It gives an exhaustive list of brands and values them. That gives you some basic idea of what brands are acceptable, because if they are in there they are probably ok, and he list zillions of brands. The actual valuations don't at all reflect ebay reality, but they are a great sanity check. For example, the book gives high values to american steel razors, and those are generally underpriced on ebay compared to German brands.
Then, I picked some brands from the book to "track" on ebay, and did a bunch of searches, and watched for a while to see what differint razors went for. I contstantly keep razors on my watch list that I don't intend to buy, just so I can get a sense of what costs what. The last hour of bidding often doubles or triples the price, so you need to track the final price, not the 3 days left to go price.
Also, pay attention to which razors seem to draw in bidders with high purchase counts. That's a blunt tool, but if a bunch of guys with 200 razor purchasers bid on a razor, that probably tells you something. I only guys with 2 razor purchasers are looking, that might tell you something too.
Also, occasionally do searches spelling things wrong. I saw a dubl duck go real cheap because the idiot seller listed it as a "Twin Duck". I think I got an ok deal on a green lizard because cattaraugus was spelled wrong.
Here's what I've bought just as anecdotal perspective, and roughly what I paid (ie what I remember) minus shipping:
1. Robeson NuBack 4/8s in great shape - $5 [seller put up a terrible picture - this was an anomoly]
2. Robeson Shuredge 6/8s in very nice shape - $50.
3. Geneva Cutlery 6/8s in very nice shape - $30
4. W&B 7/8s wedge in great shape for a 150 year old razor - $41 - had this one rescaled by another member and it is now SWEET.
5. W&B 8/8s wedge in great shape for a 150 year old razor - $40
6. Henckels 3 pin Twinworks 9/16 with inlaid scales in great shape - $38
7. Cattaraugus 5.8s green lizard - $28
8. Puma 5/8s in unused shape - $61
9. Friodur 5/8s untouched or close to it - $108.
10 Wustof 5/8 - $110
11. Engstrom 5/8 swayback untouched - $26
12. THessenBruch 5/8s in good shape - $19
This reminds me I need to stop buying razors - I'm glad I went through that exercise. :hmmm:
These razors are all great (though the lizard is still defying me on the honing, but I cracked the rest), and I've been able to see a lot of diverstiy in grinds and widths and steel feel, and I've learned a ton about honing (and developed tendinitis, but that's another issue). You can see you can get great american blades under $50, and alot of good german ones too for $50. Likewise very nice Beau Brummels go a little over that all the time. And I've seen gorgeous lesser known but great Solingen razors that haven't been used go around $50.
So do some lurking on ebay and you'll get a sense of what to look for.
Also, ask a lot of questions before you buy - does it close smoothly without rubbing the sides of the scales, any chips or scratches on the blade, etc. You can't pick everything up on the pictures they post.
Finally, get a sense for the good sellers.