Buying parts can be great...or bad. Here are a few things I look at:
You should know what part will swap over. This may be found either by buying an Interchange book or by going to one of the club sites and asking. Most people at Club Sites are great and extremely knowledgeable about the type of vehicle you have. Sites like www.nastyz28.com for 2nd gen Camaros and www.v8buick.com for Buicks. Do not depend upon the seller to know. Many times have I seen 78-81 Camaro dash pads for sale when it can be 79-81 but 78 is different.
It's kinda stupid to pay for instance 200-500 bucks for a 70-73 Camaro Spoiler when you can buy a new one for 150.00. If NOS is your thing then you should have everything, engine color, parts, etc....everything NOS.
Again, read the actual feedback, not just the numbers. Many people will place a bad comment in a neutral or positive feedback.
Most sellers are honest when it comes to parts either in the description or when asked. Ask why they are selling the parts. You don't want their junk they won't use if they are replacing it. Many time though car people will sell excess parts to help others who share the same interest and some make their money junking out cars. Ask about shipping.
Simple enough. Then Ask Questions. Then look some more.
Ask how much shipping. If it seems high it may very well be. Some will add to the shipping or add a handling fee if it doesn't bring what they wanted. Also if the seller takes it to a Pak Mail, UPS Store etc, it will be packed well but expensive.