Most likely I am considered a paper nut but this is my method.
Since I trade only via email it is easy.
I make an invoice (one mailed with the trade and one retained)for each transaction. On the invoice is a column with:
Items sent (full dewscription)
Items to receive (full description)
Date mailed
P.O. receipt (seldom used)
Web site updated (date)
Receipt acknowledged (date)
Ledger updated (date)
Trade items received (date)

It use to be that every so often I would take these sheets and update the web site and then fill in the 'Web site updated' column, update my personal collection inventory, and file the invoice in the proper folder.

Doubt this would work in a person to person trade but it works for me, or better it use to work until the family situation got in the way.

Dick Staeffler, proud father of Richard