I’m planning to start a junk hauling business. Customers will pay for us to haul out items to dispose of. Many times, I come across items in good shape that I plan to sell on the side.

Because these items are specifically purchased (and we are in fact PAID to take the entire load), I’m wondering how to account for these goods in our inventory.

The job paid for legally releases the items (“load”) to us, most of which we dispose of. But, as I noted, we can generate a good amount of revenue by selling items of value we come across—I just don’t know how to account for them. There is no “cost” associated them (when trying to calculate COGS), and I’m sure there are other accounting impacts or logistics that I’m not thinking of.

Any help would be greatly appreciated!

You don't need to have a cost to calculate a gross profit so you have revenues in two streams your "contracts" and your "sales" and you have various incidential expenses; vehicle, wages, inventory storage, etc. but I would account for dumping fees as a cost of sales item. I'm sure as your business progresses you will come across situations where there may be items you will specifically pay for because your experience will tell you they are valuable.

As to inventory, you will account for them item by item and it matters not that they have no cost. It maybe under certain tax regimes you could choose to account for them on a market value basis which may reduce your tax liability. Take professional advice