On a T account if you sold 40 units on account 3500.00(cost880) where would I post it on a t account

There will be two transactions here: One for the units and one for the sale.

So your journal entries will be:
Debit Cost of Goods Sold for 880
Credit Merchandise Inventory for 880
(to record merchandise sold)

Debit Accounts Receivable for 3500
Credit Sales Revenue for 3500
(to record a sale)

So for these two transaction you will post to 4 T accounts:
Cost of Goods Sold, the right side of the T for 880
Merchandise Inventory, the left side of the T for 880
Accounts Receivable, the right side of the T for 3500
Sales Revenue, the left side of the T for 3500