[ModelA29]

Depends on the part. A small NOS part will have the fine points guys beating a path to your mailbox to buy it. A rusty fender that needs to be crated and shipped for a lot of money will likely still be in your garage 5 years from now. The heavier/bulkier and/or more common the part the smaller the online market is since there are probably parts guys in every local club that have what you need.
As far as pricing you need to be realistic. It's not just the part. Shipping is a big part of the deal. You need to add the time it takes you to box it up and take it to the PO. The buyer needs to look at his total cost including the shipping - a $5 part with $10 shipping makes no sense so you'll be keeping that one.