Try to buy a razor made from USA, England (Sheffield), or Germany (Solingen) preferrably. These were nations with good steel production for the period. Inspect the blade before purchasing if possible. Try to avoid blades with significant or uneven hone wear (1/8in. wide or more). Avoid cracker, heavily rusted, corroded or pitted razors, especially if it is at the edge itself. Don't get one that has a "frown" -- the middle width being less than at the toe & heel. Look at the condition of the scales. If you don;t pan on replacing them, make sure they're not warped or otherwise damaged. If they're celluloid, remember that light colored celluloid could off-gas, necessitating replacement.
I maybe able to help I have a few razors Im selling and maybe able to help you out, generally anything from a member here is good so your in safe hands. regards,simon
One thing you really need to look out for, even when buying from members here. . . . . . . . . Aquisition disorder
With my slightly patriotic voice I'd like to add Sweden (Eskilstuna) to that list. There are some good Swedish razors that can be found at really decent prices!
Are you looking for an "old" razor that's a restoration project, or one that is essentially ready to use but needing some minor cleanup/polishing and a good sharpening, or one that's shave-ready?