Cool. Okay, now having that clarified (I'm sorry it wasn't clearer in the beginning), does it make sense how we do this?
When we buy a tank from a vendor, we make a bill. In the bill, we'll use the items tab and add a Fixed Asset Item for each individual tank we purchase. They are designated by volume (gallons) and their serial number in the Asset Name/Number field. The asset account I apply the tank Item to corresponds to an Asset Account with the same volume differentiation. We pay the bill with a check from our checking account.
When a customer, out of the blue, says I want to buy one of your tanks. We don't have any on hand to sell from inventory, so we use one of our rentals and sell it with a markup. What I'll do is make an invoice and use a Fixed Asset Item that corresponds to a physical tank.
When I receive payment for that invoice, I'll receive it in Quickbooks, but it does NOT modify the value of the Fixed Asset account. For this I need to create a Journal Entry (in addition to the invoice) which takes the value of the tank sold (for the dollar amount at which we purchased it), and moves that value to an "Other Income" account. This will deduct the value of the tank that has disappeared from our rental assets, and move it to a sold designation account. This account will only ever show a negative balance.
Does that all sound correct?