I'm no pro, but I have been regularly buying and selling car parts and electronics surplus online since about the time Windows 95 hit the shelves.
Here's some observations...
*Post a price.
Most forums are not a good format for an auction type of sale. Saying, "
Hit me up with offers." is generally a Fail statement unless you have an indicated price range. Want an auction? Post on eBay and link to it in the forum.
*Reply to pm's promptly and clearly.
If someone shows interest in what you are selling, entertain that interest. Someone who wants a part to repair a car is not going to be waiting weeks for a reply. Someone who is looking for an expensive upgrade is not likely to trust your credibility if you seem slow or evasive with your answers.
*If you get a couple inquiries with the same question or requests for more & better pictures, post the answer publicly in the thread.
*Pictures in good lighting that show off great condition and highlight and explain minor defects are better than dark, blurry pictures that are ambiguous.
*Accept that "rare items" are often rare because they are undesirable to most people. If you want top dollar for it, it will be a slow sale.
*Do not take replies to your post personally or get in a flame war in your own thread. If you have a serious troll taking over your thread, either contact a moderator, or drop that thread.
itsnotanova is a great example of how to do it right:
http://986forum.com/forums/boxster-parts-sale-wanted/52469-parting-out-whole-lot-boxsters.html
These threads, maybe not so right:
http://986forum.com/forums/boxster-parts-sale-wanted/55004-zeintop-sale.html
http://986forum.com/forums/boxster-parts-sale-wanted/55963-porsche-996-986-boxster-cassette-drawer-insert-cassette-storage-leather-rare.html