Reasonable on 03 January 2011
None. While quality can help sales it can also clearly be irrelivent in the face of brand or marketing or circumstances. Plenty of low quality games (and films) sell lots. A fair number of clearly better quality (if by quality you mean better made based on the accepted standards of the medium) titles don't sell lots.
Therefore, sales quite clearly don't directly equate with quality.
Try to be reasonable... its easier than you think...
