My number one rule is to never take their first offer. On this one, though, I would make an exception.
I worked in the nutritional supplement industry for many years, before quitting a few month ago to focus on my web biz. HCG was the last fad I experienced, and it was already dying down some before I left. People were amazed by how much weight they were losing, but the kicker was that you had to go down to around 500 calories per day. What obese person couldn't lose a lot of weight on a 500-calorie-a-day diet? And what nutritional deficiencies does a 500-calorie-a-day diet cause? With these questions, how long do you think "HCG" will be getting that many exact searches?
There are only a few supplement fads have any staying power. Off hand, creatine and N.O. boosters are the only ones I can think of, but I could be forgetting some. Ephedra did as well, but it was banned because of its striking similarities to meth and its propensity to induce heart attacks.
Anyway, congrats on a nice sale!