when you sell a partial or entire interest, the PTP will provide you with supplemental sales schedule which should provide the info needed to record the sale (except for a possible wash sale) and any ordinary income recapture IRC 751. (some PTPs don't have any others have a lot) The PTP probably uses FIFO. Can you use something else? well you have to tell the broker what specific lots you are selling and then the broker would have to send that info to the partnership. will the partnership reporting honor this? Don't know. Recording a partial sale could be messy and I have never dealt with such a situation. The other thing is you may have to watch for is a wash sale. The broker may report a wash sale because it only knows what you paid originally not what your true tax basis is so even if no wash sale their reporting will need adjustment.
by the way typically older lots have more ordinary income recapture than newer lots - this IRC 751 recapture is the depreciation taken by the PTP as allocated to the various units sold, if any. There is no way to know this amount until you get the k-1 package.
