(Ok, I am replying to a tech thread, I'll try not to get it locked..)
eBay intercoolers are very hit and miss, from what I've seen. Just like the turbo manifolds, it all comes down to how good the builder was and how cheap the price is versus how much he felt it was worth. When it all comes down to it, you can look at the construction of the endtanks and core from the pictures and judge the quality of work that way. I've seen turbo 323s in Puerto Rico using $200 eBay intercoolers runing 10 second quartermiles.
My advice? While you can never say ANY intercooler is better than no intercooler, the GReddy kit really wakes up with an intercooler. Make sure you get a kit that has nice endtanks that aren't going to be road blocks and and get some nice smooth plubming and you won't regret it.