Fact is, after all fixed costs are allocated (attributed) and variable costs are added to the actual "cost" of the domain to the registrar, almost all domain sales are at a loss.
The goal of the registrar is for you to buy other services from the same place you have your domains registered, and to sell the domains to folks who might not be at the registrar currently so create an account there in order to accept the free push.
It is true I would be a little cautious around deals that "look to good to be true", however.
I was just keeping in mind that registrars pay $6 to Verisign for .com and $5.50 for .net. Anything above is the margin. Granted, the margin may be low and cut by other costs but at least there is a margin...