AwesomeGuildStore has an option that can output the information (name, price, time of purchase, etc.) to your chat as a gold system message whenever you buy something.

Take a look at the conditions that generate that. Probably something like on item added combined with if the menu is the guild store. Then, create a saved variable table and add a string entry to it with this information each time you buy something, instead of outputting to chat.

Use normal saved variables for the table if you want to track separately per character, or account wide if you want to combine them all together.