What marketplace was it? Was it something like Myrf's shop? Or was it like a global shop?

Well Myrf is actually a nickname for one of the members on the server, EmperorMyrf. If you do /town spawn Empire then go straight ahead (I think) you will reach a marketplace made by Myrf that sells stuff and buys stuff, you don't see a lot of chest shops that allows you to sell to it so I'd recommend you checkout Empire's marketplace first before anything else.

Ok so I found one and clicked on a sign and I bought something and it gave him my money, (quite cool though i don't understand how it works) but now I dont know how to sell stuff back to him? Help please!

Although selling back to someone only works if the sign is set up so that they buy items from other users instead of just selling.

You can check whether someone buys something as the third line on the sign will look something like this:
"B 26 :26 S"

I'm glad to help however I can.

There's some more stuff that I thought you would like to know about how the shop works.

Here's data values about the different items in Minecraft: http://www.minecraftwiki.net/wiki/Data_values

Now say that you were wanting to sell something like... Lapis (The drop, not the block or ore). You might think that all you have to do is put "Lapis Lazuli" as the name for the item to sell. This is actually not true, if you do this you'll instead have the shop think that you're wanting to sell the block, and any Lapis drop you put in the chest won't sell.

The way around this is data values. You'll want to do this instead "351:4". This is because Lapis is a dye in Minecraft, and it has a strange data value. This data value means it's number 4 of the item "351", which is the "dye" item. While the sign may say something like "Ink Sac: 4" or "351:4", don't worry. All you'd have to do is put a sign by it saying "Lapis Lazuli" to let people know what it is.

Another example is say you want to sell Pine Planks, or Jungle Planks. The regular Planks data value is "5". Looking at the extra data value for which one is Pine and Jungle, you'll see that Pine is "5:1" and Jungle is "5:3".

Any thing that has different types like this can be sold in this method. Things that can be included in this are:
Logs, planks, leaves, coal/charcoal, saplings, wool, dyes, slabs/double slabs, sandstone, and stone brick.

That's because that's one of the ones you don't have to do the "#:#" for on the data value.

Simply put "Iron Ingot" on it, and it'll be fine.

Thanks!