The only realistic thing that you could do is bumping your thread. It will take awhile for your shop to take off. Also, remember to bump within the guidelines/rules.
Yes! If you don't mind advertising in your signature, you should do so! Just post the link in your signature.
Otherwise, just bump up your thread by typing "Bump" and posting it on your thread. It will send your Thread to the top of the section making it visible.
If your selling something useless, and easily found in-game, (E.G Normal flowers) nobody will buy it! Howver if you are selling something rare ish (Hybrids, Unorderables) people will buy it!
People will see your thread. You've just got to be patient and stick with it, because it's likely that people just don't have them. Or some people will have them, but don't want to give them up.
As long as you keep your thread on the first page, I can guarantee that people will see it. Being at the top of the page doesn't help anymore than being in the middle, except it'll be displayed on the main page.
Bump your thread every 2-3 hours.
Edit: You could also advertise it in your signature with a link to the thread.