https://geekhack.org/index.php?topic=68372.0Great resource to check out to get started on GH
You can buy and sell things (personal things) in the classified section of GH although it would not be the place for selling a product or group buy, that is reserved for the IC and GB sections
Just be nice, don't bother people too much, good thing to note, rather than replying a new comment everytime for update, you can just edit your original post. Also don't spam around too much, and especially for posts in the IC/GB section, people like to see some time and effort put into thought with something like a prototype, render, etc.
Welcome to the hobby!