The best evidence would be sales receipts and, depending upon the circumstances, the IRS may not accept anything less.
However, pictures of the items and handwritten notes as to the prices paid and dates of purchase will most likely suffice.
Importantly, you cannot use losses on the sales of items held for personal use to offset gains on other items also held for personal use. In other words, you have to report the gains (and potentially pay capital gains tax on those gains) but cannot deduct the losses.
