Is there a website where you can look up how many copies were sold of specific albums?

The Soundscan website... but you have to pay (BIG BUCKS).

what he said. plus the data is incomplete because of all the records sold pre-soundscan and suchlike

i think the RIAA site will give you numbers as long as its gone Gold, don't know how much that helps you though.

The curiosity mainly comes from seeing how many albums
an old "obscure" band that is now considered seminal
sold while they were a band. For example, Can.
I wonder if the new reissues are selling more copies
than they were when they were originally released.

soundscan will be no good to you. i don't think a specific site for what you're after exists at all

www.riaa.com does tell you various dates of gold albums, so frinstance there are about ten certifications for Thriller going up to its current 27(?) times platinum status.

Until Soundscan (which has its own inaccuracies), there was no way of determining sales at all. Why would there be? Despite the essentially private nature of this information in itself, why would labels want to offer information about sales to the view of the artists that they were cheating?

Can is additionally problematic because of the international nature of their sales. Originally, the records sold reasonably well in continental Europe, less so in the UK and barely at all in North America. As their sales declined in continental Europe, they rose somewhat in the UK (the relative late "I Want More" was something of a hit) - still nothing in North America.

Their sales in America are much better today than in the past in every sense. In the UK, I'd imagine that the earlier, more crucial albums (the first six, let's say) perform relatively better than they did originally, the later albums somewhat worse. In Germany, were they actually "shifted units," they probably don't do as well in actual numbers, but perform pretty well considering their older, non-touring, strictly reissue status might imply.

As a gross generalization, Can record sales are much better than one would have ever imagined they would be in some random year after their break-up, like 1986.

It may impress people to learn that "obscure" bands such as Swell Maps or the Raincoats sold well enough in their day that they would have made the charts ten years later if they'd sold the same amount at that point in time. (For instance, the Red Crayola's "Kangaroo?" sold more than 10,000 copies on vinyl.) That's because in the UK it takes far fewer records to make the charts than it used to. Part of this is due to declining sales, but part is also due to the substantially larger number of releases available. There are something like 20 times (!) the number of releases "easily" available per annum than there were in the 1980's. Many people don't realize that - outside of a few examples - there was no such thing as a "reissue" label in 1983 really, and that things that would seem strange today were the norm then. (For instance, it would be weird for any of the "real" VU albums to be out of print today, but in the 1980s it was the norm for at least half the catalog to be OOP at any time, and it was a much smaller catalog then too!)

I bet the Can CD's are all selling much better than the original LP's - for on thing, they are now being marketed internationally by essentially the same people - i.e. you can actually buy them, they're available. Plus, all those years, the word gets out...the other act that sold much better in CD format is Jack Bruce, whose wonderful LP's sold not well at all when new - yet Polygram made CD's and one full set sold out, and they have now been re-issued as re-mastered Cd's - come to think of it, same thing is happening with Can! (Great music, Can, Jack Bruce...)