I used to collect gold coins both ouncer and various fractionals, Eventhough it is a quite tedious job of sorting, i never lost the tract of knowing the price how much i bought. I register them in an spreadsheet as a part of my savings plan.
Off course i know, i can't hold them forever and if i am going to sell them in the near future, i'd just enter present market value in the spreadsheet, then i can check easily know how much i gained or probably lost. I used to have lot of collections coins, stamps, banknotes, watches, medals, jewelries, precious stones/gems,whisky's, plates, jars, paintings and more. i never jumble in the prices because i am enjoying them. I also scanned or take photos of their receipts, calling cards, stores information.
If you personally assemble a collection, i am sure you will never forget the price.
Checking auctions, web prices, ebays and local sellers group may give you instant information of current market of your items.
However, IMO, a plan to sell gold coins in short period, you are just prone to losses. Leave this job to the dealers because this what they do daily.