The Opt Qty field (also know as the Optimal Quantity) is used to list products onto your buylist.
This is the total quantity you would optimally like to have at any time in your admin for a product.
If we have a look at the image above, this is the qty information for a product called Drowned Catacomb.
For the NM-Mint condition, the current qty is 0, but the opt qty is 8. Therefore, this admin will have 8 products that are on the buylist as it means that the admin would like to have 8 of this product at any one time.
For the Light Play condition, the admin has 6 in qty and the opt qty is 4. Therefor, this admin will have 0 products are on the buylist. The reason is, the admin wants to have an optimal quantity of 4 of the Light Play card at any time in their admin. As they already have 6 in their admin, they do not need to buy anymore, thus these cards won't appear on the buylist.
For more information on how to list products to your buylist, please click here.