Not sure, but steamspy does it for steam. Developers have said its numbers are very accurate:

http://www.gamasutra.com/view/news/291530/Devs_weigh_in_on_the_accuracy_and_utility_of_Steam_Spy.php

https://steamspy.com/about

Well until Rockstar themselves say anything about it I guess we simply don't. 

Yeah, that's why I didn't include that possibility in my initial question.

Checking bestseeller lists unfortunately only gives a rough indication at best (a somewhat better one if any of the games sales on the all-time lists can be tracked or have been revealed in the past.) Combined with relevant ratios you get educated guesses at best. Wich is interesting, but also a lot of work for mediocre results.

Updating digital numbers as they get released, just so they are at least represented might be a good compromise.

I always assumed they sniffed data packets or something? I really don't know what I'm talking about though. ;P

Unfortunately while Steamspy is the best digital tracker we have for PC it really can't be used as an indicator for consoles because of the diffrent audiences/buying habits/overhelming digital ratio on PC.

Edit: So apparently Steam Spy is getting their data from user profiles and extrapolating from a sample size. Since it's all digital there's no trouble with having to account for physical like there would be on console (Switch shows you what friends play for example, but there's no way to tell if it's digital or physical). I guess if you had a sufficiently big and varied sample size of users willing to make their profile public on each console you could extrapolate and then deduct physical from the total and arrive at a ballpark. No idea how to achieve that in praxis though.

Never suggested they could. Point was, if they can do it for steam, that has to be a way to do it for consoles.

Nah, I didn't think you were, I was just lamenting. See my edit above for how SteamSpy does it and why that's a bit more difficut to do on console. ;)

Very unlikely

Would like the database to be update first though

I know that with Steam sales figures can be estimated by checking people's public profiles, but I'm not sure if anything like that is possible on other platforms.

More like will this site ever track physical sales correctly.

PSN and XBL doesn't have APIs which support collecting data about how many users played some game or have it on their accounts. Steam does have this. That is why SteamSpy exists. So I don't think that it is even possible to track digital sales on consoles as of now

Pretty hard with how much more opaque digital stores are, but VGC will need to figure something out if they don't want to lose relevance into the future.