To track the gain you must first have the cost or basis entered as an asset. Then when you sell what cost you $7000 for $10000 you post a sales receipt of 10k, and for the items sold you will post 7k to an item that directly affects the asdet and 3k to an income account called something like Gain on Asset Sales.
Long or short, capital or ordinary is not specifically accounting but is a tax issue after the fact. Usually Schedule D although there may be a different form for businesses.
If you do this regularly and it is your business model you could consider tracking the stock as inventory except QB won't easily let you choose which units of stock to record the gain on if you have batches at different basis
