ManBeef.com is a very well-organized and superficially convincing hoax
According to www.ManBeef.com, the company has been in operation for 19 years, providing quality cuts of meat for those human-flesh gourmets. The website includes Processing and Policy pages that make the concept almost believable, provided one does not actually do any searching for the truth beneath the polished surface. The FAQ gives reasonable answers to the casual websurfer, though the very first answer is an outright lie.
Q. Do you really use real human meat?
A. Yes, all of our products are made with 100% human meat. We do not add any other meats or "fillers" to any of our products. When you order from us, you can be assured that you will only receive the highest quality human meat. To find out more about our meat, check out our Processing page.
Checking out ManBeef.com at Snopes2.com reveals that the whole concept is an elaborate fiction traced back to a 16-year old North Carolina boy.
Cannibalism is undoubtedly the strongest taboo in modern society, and in a country where most people turn up their noses in digust at the thought of eating horsemeat or duck eggs, the sales potential for human meat is negligible. Aside from that pesky marketing problem, however, is the inconvenient fact that it simply isn't legal to sell human flesh for consumption (by other humans or anything else) anywhere in the United States (or to export it out of the USA), no matter how "fresh" or carefully packaged it may be. (The recent scares over Mad Cow Disease have led people to object to the practice of feeding livestock ground-up parts of other livestock, so the notion of humans eating other humans just isn't going to fly.) - From Snopes2.com
Additionally, Snopes2.com states that the neither the USDA nor the FDA sanction the selling and consuming of human flesh, and it is illegal to sell any meat product without the proper inspection certification.
Information taken from www.manbeef.com and http://www.snopes.com/inboxer/hoaxes/manbeef.htm