You certainly can!
I'm sorry, I've not put together a guide on creating shops signs yet. They can be created anywhere.
Briefly, the shop sign format is:
Line 1: [Item]
Line 2: Buy 10 for 1e
Line 3: Sell 20 for 1e
Line 4:
The first line is the name of the item, surrounded by [ ] e.g. [Bread]
The second line is the amount of the item you are selling, and the price. Read as if you were the customer. Buy 10 for 1e = your shop offering 10 bread in exchange for 1 emerald.
The third line is the same, but where you would offer to buy their item(s) from them. Sell 20 for 1e = Your shop offering to give 1 Emerald for every 20 bread the player gives.
The fourth line should be left blank. If the sign is set up correctly, this will automatically fill in with your name.
Capitalisation is important. You may find that the item you are trying to sell is not set up correctly in the shop system (it doesn't have a name assigned); if this is the case just send me a message with the item id and the name of the item. I'll add it. You can just use the Item ID, though. e.g. [43]
Hope that makes sense