Always be suspicious of anyone offering a product for a price that seems too good to be true. More often than not, it is too good to be true. I always get suspicious if someone is selling me product for more than a 20% discount from any other store. Unless this "deal" comes from a dealer whom I have dealt with before I will walk away and purchase it somewhere else.
For example, a user on DPReview found a Canon 5D for sale for $1,895. The original price for the Canon 5D is $3,300. Five months after release and most reputable dealers are now selling the 5D for ~$2,995. The $1,895 price should immediately raise a concern on anyone's mind.
So before purchasing any product online from a vendor you do not know, make sure to first check one or two comparison shopping sites and to review the reseller's ratings.
Two of my favorite comparison shopping sites are: