Hello. Near the top left is "Board Index" (if you're reading this, you can see it directly above). Click on that, and you will get the index page, which has every section and subsection clearly named (with direct click links), which I think is fairly self-explanatory (if you're not sure what a section/subsection is about, just click on it and have a look!).
The section for buying and selling guitars (a subsection of the "Luthiers" section) has the rules for selling instruments at the top: