Customer trade-in item and how to offset new purchase in a right way

I'm trying to do this right, and I'm only on Quickbooks Online Essential so I might be limited by the features due to my subscription.

Today, i've manually keep track of my inventory, and whenever i purchase goods, i make sure expenses to the supplier are accounted for in the "Inventory - Product" chart of accounts, when I make a sales, I create a journal by credit the "Inventory - Product" and debit the "Cost of Sales - Product" chart of account. 

I do have request that wanted to trade-in their item, for example, I might be selling an item cost $1,000 but customer would want to trade-in their existing items which I will take at $200, and how do I account for this and issue them an invoice with the $1,000 items but have a separate line states the trade-in items at $200, so nett nett he/she is paying the difference on the invoice which is $1,000 - $200 = $800?

And this $200 my thought of it should be moved into another chart of account, says "Inventory - Trade-in", so when i make a sales of this trade-in items, i can create a journal to credit this "Inventory - Trade-in" chart of account, and debit the "Cost of Sales - Trade-in" chart of account.

My business is not high in volume hence i'm still doing this the way I described, I hope to account trade-in item as accurate as possible so would really appreciate some advise

many thanks