UGHHHH
a little more detail would go a long way. Not that I personally would've responded 10hrs ago either, since I wasn't here then... but.. anyway.
Regardless. - Method 1
Open shop (door plaque reads closed or open - you want open)
Wait for a customer to spawn between I believe 7am and 7pm
Get order
Make the tool head of order
Combine it with the proper parts - It should tell you in the bottom right corner "You have created (Metal) (item)"
Give to customer.
You've sold an item.
Method 2
Make a bunch of various tool heads
Make a bunch of correct combinations with proper parts (should see notification if it is correct - if it isn't, its not able to be sold as is.)
Open shop, wait for customers
Attempt to match items to Order - If you're out of money and out of materials and don't have the item, you can turn them away - or if you're feeling gutsy, you can take an item to the anvil, break it apart, take the tool head, and smelt it back down for new material, and reshape the material into the ordered item - reassemble it with the right parts.
You've sold items (probably)
There's no lower-gains method(s) of selling items for stock prices atm. They must be sold to customers of some kind.
There've been suggestions proposing having a way to sell it to another store or npc for a reduced price - one you could take batches of items to and immediately pawn them for coin so you don't have to wait for someone to come specifically asking for it.