Here is a image outlining how trade signs work for selling items to players. See attached.
Here is a more detailed description -
Before you create a trade sign you need: a: A spare sign b: enough of the item you want to sell in your inventory to place into the sign to sell it. Eg to sell 20 cobble, you need to be carrying 20 cobble. c: to remember to remove/break the sign once it runs out of stock in order to get your money out of the sign.
Signs are basically broken up into 5 parts on 3 lines. Place the sign on the wall and type in the following details - Line 1 - use the keyword [trade] to tell server its a player trading sign. Eg: [trade] Line 2 - put in a dollar $1 amount to charge players each time they buy from the sign. Eg: $1 Line 3 - has 3 parts. First part - number of items to sell each time they click to buy from sign. Eg 1 for a single block or 64 for a full stack of blocks Second part - the item name or Item CODE of the item you are selling, third part the total number of items you want to put into the sign to sell. Eg 128. The "128" part of the sign will automatically decrease on the sign each time an item sells (eg sell one and it will read 127)
So for example to put 200 cobble up for sale, at $1 each block, selling 1 at a time - create the following sign: [trade] $1 1 cobble:200
Or for example to sell full (64) stacks of cobble for $70 a stack, and you have 900 blocks to sell - create the following sign: [trade] $70 64 cobble:900
If the stock is used up you can "refill" the sign by holding the item in your hand and right clicking the sign.
You can also create BUYING trade signs - these work in reverse swap the first and second line. You need to have enough money in your balance for the value of the blocks you want to buy. Each time a player clicks it will pay them your per sale amount, and remove that many items from their inventory. Note: Don't create trade signs that buy more than your inventory can hold, otherwise when you go to collect the goodies you may loose some.
Eg: Line one [trade] Line two #(units of each sale) itemname(or id) : total already purchased
Line three $price : amount of money to put in sign (eg $200:1000)
Eg: [trade] 1 diamond $200:1000
Thats the basics, Trade signs do a few other funky things I don't have details of, feel free to experiment.