Both the courts and the FTC have actually ruled on this issue. The legal term is "puffery" - advertising a subjective feature that really cannot be quantified. On the FTC web site they give a good example:
Simply the Best?
What is the “best” video game, the “coolest” kind of shoe, the “hippest” mp3 player?
Your answers may be different than your best friend’s, and from someone your age who lives a few states away. Those words mean something different to everyone, and advertisers use words like these to sell their product.
It’s known as “puffery,” because these terms “puff up” products. Puffery usually isn’t considered misleading, because it’s a pretty obvious exaggeration.
When a company gets into facts that can be checked, though, the claims have to be true. Consider these two statements:
(a) “The best-tasting juice in America!”
(b) “75% of people prefer our juice.”
Which one would require proof? If you said (b), you’re right. The company would have to be able to show the survey that they took to
make the claim.
Therefore, misleading potential customers about your sales volume is not "puffery". It is a quantifiable fact and subject to verification.