How to Tell Minecraft Cape Link Legitimacy?

Buy?

There are two ways to get a cape.
A - Get a real one added to your account by Mojang, as a reward or other promotion.
B - Use a mod to add one. Normally only you can see it, but with some mods (like Mccapes) anyone with the same mod, can see them.

There's a multitude of free mods that can give you one. But anyone asking for payment is either directly scamming you or trying to sell something you could have got for free, and that probably doesn't do quite what you thought it would (ie, not making it clear that no one else will be able to see it). Anyone that claims they can give you a 'real' one, like in type A above, is a scammer.