Most of us are not going to have receipts from all the models we have purchased over the years, but what i am going to do is document what the kit that I am selling, whether it's unbuilt or built, cost new and what it is selling for now. If built I am also going to document the cost of aftermarket parts, paint or anything used in building it. Plus you can deduct the cost of shipping so you won't be paying taxes on that. Yes it will take a little more work to log all the costs but for most of us, we aren't really making any money on most of the stuff we sell. For me, I just hope to come close to breaking even and clear out some space. Just my 2 cents worth.