Crafted items usually go by the price of their raw materials. Say, a regular diamond helm should sell for about 5*200 = 1000 Noobz. For enchanted items, people usually auction them. Most enchanted diamond tools sell for anywhere between 4k (low-end or single enchants) to 30k (3 or 4 enchants with 1 or 2 of them being rare like ST or EffV).

When selling to a player, either buyer or seller would usually initiate it: "Buying this item" or "Selling this item" on global chat. People respond "Yes, I'd like that!" on global chat, but sometimes it's best to use private messaging (/msg <playername>) unless it's an auction. An initial price is set, which may go up or down until the two parties are satisfied. When a deal is made, they go somewhere isolated (so people close by won't steal). Some buyers prefer you hold the item in your hand first. Some buyers would like you to hand it to them first so they can verify that it is unused/in good condition. Some sellers would want the buyer to pay first. Either way, the buyer does /money pay <seller name> <price> and the seller tosses the merchandise at the buyer's feet. Finally, thanks are given and the transaction is completed.

I was gonna say, do "/shop list" for prices, but it's down right now . The next advice would then be to check prices at the spawn shops (/spawn and to the right). I'd look up the buying price for the raw materials to get an idea on how much I would sell my merchandise.