After using .biz for one of our main projects after its launch a couple years ago, I have found several problems with it:
Your average Joe Surfer is accustomed to .com for everything, especially ecom sites. Building an ecom site using a second-rate extension like .biz, can raise doubts by visitors who may see the website as an upstart or inferior to its rivals.
the extension is an informal/slang term, which IMHO doesn't give a professional aura to bizness
stickiness --- confusion over the extension leads to fewer repeats
spam in the .biz TLD - Neulevel continues to dump .biz domains at lower prices making it very attractive for spam and fly-by-nights
<off-the-wall>there is something just more aesthetically pleasing to .com over .biz... take a look at 2 similar listings in the search engines and you should see what I mean. Whether it's .com's smoother lines as compared to .biz or what I can't entirely put my finger on it.</off-the-wall>