I figured it'd be a good idea to have a thread where vintage collectors could share knowledge and experience to prevent other vintage collectors from getting ripped off.
I only have two bits of experience to relate for now:
Original weapons vs. repro weapons--I've only actually seen guns and spears and the like, I've never actually seen a repro lightsaber or cape. When comparing repro to original, the most obvious sign I've notice is the repro are sometimes larger and shinier; but if you're in a store and don't have a basis for comparison then ask to see the figure and weapon. If the weapon is made of really hard plastic, which would seem to snap easily, then it is probably a repro. These are the repro weapons you can buy from places like Brian's Toys in bulk and some unscrupulous collectors might try to pass off as the real thing. The vintage weapon are made of a softer material (though not nearly as soft as the new toys) and have some give to them when you bend them.
Check the manufacturer date on the figure/vehicle before purchase--I was at a comic book store a few years back and saw a vintage Chewie for $15, not thinking I bought it and when I got it home I noticed "LFL 1995 Kenner" on the bottom of his foot! It was the Classic TRU 4-pack version! Vintage figures usually have the manufacture date along the leg somewhere (except for small figures like Wickett who has his on his back). However the only vintage figures that have been rereleased recently in large numbers have been the 4-pack: Luke, Vader, Han Solo (small head) and Chewie. The differences are obvious when you compare these new ones to their vintage counterparts side-by-side, but when you see one alone in a store you really should check the manufacturer's date.
Well, that's all I can relate for now. If anyone out there has some advise for your fellow vintage collectors then please post it here.