Ok, there isn't really rules for selling your crap, but at least i can give you some guidelines to help with the process.

1. List the parts you want to sell.
2. List the price for said parts.
3. Detail your parts, since this is an exterior board, write the color of the part you're selling.
4. Part numbers are nice to have, you would not believe the indescrepencies between parts.
5. If you are parting out your entire car its understandable that you don't want to list everything to sell, so instead list the parts you have already sold. That way nobody keeps asking for your front S4 bumper over and over.
6. Audiforums.com is not responsible for lost or stolen property, so don't make deals with members that don't have less than 100 posts, they probably aren't reputable.