It seems like you cannot directly try to sell products by saying "Buy this product" on the forums. However you can indirectly have links to what you are trying to sell.
If the product is software and it is up to TGC standards a proposal can be setup for the product.
If the product is software that will not be related to TGC or it is a misc. product than you can have links in your sig to the site you are selling from. But if you have a link in your sig you can't say "Buy" anywhere near it.
You can also indirectly ask if the product is reasonably acceptable to be sold, on the forums by posting something like "How do people like my product". If you do this then people will look at your product and give input on the product. Some people then may want to buy the product as a result of getting a look at it.
For you to get people to buy indirectly:
1)Use proper grammar when posting
2)Do not mention buy in the post at any time, just have a link for it at the main web site.
3)Answer people's questions truthfully
4)When a mod says stop, stop
5)Be creative.
[edit] With regards to the following who have posted, use the exaples here with extreme caution.[/edit]
[edit] The term product in this post is hereby meaning "of relation to anything that has to do with programming/game development"[/edit]
1tg46 is my name coding is my game.