I save PDFs of the invoices. That's nowhere near as good as a database if I ever have to dig up the purchase record, but it takes minimal time. It's a compromise between maintaining a good database and not having a local record at all, which isn't much good if one buys something at a store that later closes.

I learned I want the local record because a long time ago, at some other 3D shop, I bought a bunch of wood textures, and didn't save an invoice anywhere I could find it. And when I realized I wanted to have a proof of license before I did anything with the textures, I couldn't even remember which shop I bought them at, and couldn't track it down. Oops.

I have more elaborate technical-side info-gathering, unfinished as yet, with the end of trying to make sure that I can find my stuff when i want to build a scene. These days I always download a copy of the product's web page, and a second, more conveniently labelled and organized copy of all the product images, to store with the model files, so I can glance through my directories and have some reference to what the heck the product was. (To which end, I bought Directory Opus as a replacement for Windows File Explorer, as it's much more reliable about showing images on folder icons.)

I try to keep notes of what products went into a render, too, so I have some hope of rebuilding it if I want to adjust it after the original doesn't work.

My system isn't designed for speed and elegance in recovering billing records. It's designed to be simple and fast enough at record-capturing to ensure that I actually perform this chore at purchase time, instead of putting it off. I'll have the records if I really need them, but I don't expect to need them often, or maybe at all.

So I put my effort at efficient identification into designing my model collections.