If you're going to do something, do it right. You often hear about how it's a no-no to only use winter tires in the front or rear wheels. I see what you want to do as something similar....too much grip onto one axle may create a dangerous situation on/off the track. If it were me, I'd go with all 4's. If money's tight, use the OEM tires, wear them out and by the time you do, hopefully you've got a few extra pennies saved to spring for all 4.
As for going with Tire Trends vs. eBay. Sure TT costs more, but the warranty, and knowing you're getting a good new tire for each side of the car is worth a bit to me. Going eBay is cheaper, but many people have been screwed by eBay sellers, and buying something that connects car to road is no minor matter. Damaged tires or after sales support is nil on evil bay....I've never purchased anything on eBay....ever. Not about to start now either.