I use a simple Excel sheet to keep an inventory. I keep these data of each prop I buy:
Movie / Series / Franchise
Original Creator of the Replica
Date of purchase
Price in Euro
Paid through (PayPal, Money Transfert, traded for X)
Place of Purchase (The RPF, COW, eBay, Webshop, etc...)
Bought from (Nickname on forum or eBay
Full name of seller
E-mail of seller
I also have a color code: props I'm willing to sell are in blue. Once sold, I strikethrough the line, but keep it in the excel as reference.
I also have a "status" colum in front of each so I can see which props are ordered / paid for / delivered, etc...