Left click on the sign to buy, right click to sell. You may only buy from signs that have B price and sell to those that have an S price. Some have both.
if you want to create a shop type on a sign on top of a chest and write <name> <enter> <amount> <enter> <B or S price> <enter> <item you want to sell or get from people selling to your shop>.
Actually you can skip your name and skip the b or s, it will do the rest like this: BLANK LINE 5 10 wood or if you also want to buy items back (3 would be the buy back price): BLANK LINE 5 10:3 wood
Are there certain items that cannot be sold? I tried to make a sign for mushroom stew and it said it was an invalid item id.
Some items have strange names, its a game glitch. Try looking in the official store (/store) for the name, but I am not sure if that is in the store or not (it should be).
Most people will not bother to tell you to go and read the Rupees & Shops portion of the guide... That is why they ignored you. It is spelled out rather plainly.
Good point, the guide doesn't actually mention /iteminfo or the Town Store. We may need to add that information. Typing /iteminfo while holding the object in your hand will give you the item ID. However, the IDs used in the Town Store (/store) are more accurate and should be used for reference.
That is ^ Definitely agree that this information should be added to the rupees/store section of the guide.