Another idea you could possibly use is an inflation calculator like this one :https://www.usinflationcalculator.com/
Let say your parents bought you a toy in 1975 for 5 dollars and you just sold it on ebay for 40 dollars. If you use what they would have spent on that toy today it would be a little over 27 dollars according to the inflation of the dollar so that could be the basis of the value. Then after ebay and shipping fees, you might net 0 as profit.
