There isn't any way to check the sales figures on these things, is there?
Actually, I wouldn't be surprised if the Gram Parsons cult has pushed Sweetheart's sales over time beyond the first few classics, which were released when, comparatively speaking, not all that many people bought LP's. My guess is that since 1970 or so, the biggest sellers by the original lineup have been greatest-hits compilations. But this is all pure speculation.
Oh yeah, there's also the question of whether it's the best. I love Sweetheart, but I'd take Mr. Tambourine Man if I could only keep one.