Tirebuyer.com is owned by American Tire Distributors, the largest tire distributor in the US. It's their answer to TireRack.com. As with just about everything, they might be cheaper than other places, and might not...do your comparison shopping.
I have a friend who owns a tire shop, and there are times he can get tires cheaper from Tire Rack than from the distributor directly. There are other times that the local shop might have gotten a special buy on a certain model, and can sell them cheaper than anywhere else. Keep in mind that the margins on tires is not all that high (maybe $5-$10/tire) so don't assume that local shops are significantly more expensive than online dealers.