The collection inventory software I use has an entry for whomever I obtained the piece from. Long ago, I used to lightly write in pencil, the name of the dealer I purchased an item from, plus the price paid and venue, on one of the box flaps. I don't think I've run across any of those in the current inventory effort (itself just reaching 480 pieces), although it may be that I wasn't recording that info and the pieces are in the database as placeholder entries copied from very incomplete paper records awaiting more complete info. Here and there I have pieces purchased from private collectors, and that info will be entered as the source.
I did recently run across an email exchange from the early 2000s (I have archives going back to 1997) detailing my purchase of a K-Line clear-shell MP-15 from our very own Alan Miller (paid via money order, which the USPS took on the scenic route before getting it to him) . So now I have the info on that piece when it gets inventoried 
---PCJ
