I'm glad you were able to track your purchases, but it would certainly not be "easy" for me. Firstly, I have bought several high value watches for cash. I have also made a number of trades for high value watches, both in person and at a distance. For the trades done remotely, I would have to find forum messages or emails, which might or might not have been deleted. The same would apparently be true for watches that I bought using bank transfer, Transferwise, PP F&F, etc. We are talking about many dozens of watches. OF messages are saved, but on other forums, I frequently delete messages because my mailbox fills up. Finding emails from 15 years ago might also not be easy. For the in-person trades, obviously I have no records at all. I do keep a spreadsheet for my own records, so I can certainly put a reasonable value on what I "paid" for each watch, but documenting them would not be easy in many cases.
Moreover, some watches have been upgraded and/or restored, at significant cost. I suppose that cost might be included in the cost basis, although routine maintenance (like a watch service) would not. Good luck separating all that out. Then you also need to document the eBay fees, which have traditionally been billed separately at the end of each month. It's a lot of trouble to go through just to show that you took a small loss.