Admit it, you HAVE looked for parts for your TT on eBay hoping to stumble accross some kinda good deal. Yes, that turbo kit that lets you make 400whp for $100 must be out there somewhere!
Unfortunately, the reality of eBay is that 90% of the parts that are supposed to be for the TT suck. However, there are a few gems amongs the pile of garbage. I'll go over a list of some potential items you may have raised an eyebrow at:
Tail/headlight tint film - People have used this successfully. I'd still be leary of cheap headlight tint the same way as cheap window tint. I've heard that some of these don't fit the lights worth crap. And of course, you might end up with issues with bubbles during installation, and fading/peeling in the long run. I'd hate to have to take a razor to my lights to clean off adhesive. Personally, if I wanted my lights tinted, I'd hit up these guys:
Their tint jobs look amazing: http://www.jonstintedtails.com/gallery/index.php?list=Custom%20Tinting%20Gallery/Audi&page=2
The eBay single frame bumper:
A guy on Vortex actually bought one and received it after several weeks (looks like the Polish guys are legit). He said while it's not the greatest finish quality, he can fix it up. It also comes with an A6 grill which is smaller than the typical A8 grill. I'd stay away since I would never want a fiberglass front bumper. If a big rock hits it, you could easily get a spider-cracked gel coat.
The eBay FWD downpipe:
Here is a gem! These work great and cost way less than other downpipes. Plenty of people run these on VW's without issue. Unfortunately, there are none for Quattros!
Audi Style Wheels:
I've never heard of anybody buying these. Personally, I wouldn't. Chances are, they are gravity cast wheels that weigh about as much as Fred Flintstone's wheels. That and maybe a half dozen potholes later riding on low profile tires and you'll have a set of stop sign shaped rims. Who knows, perhaps they're quality! But I wouldn't buy them.
Golf/Jetta MkIV intercoolers will fit the TT with some work. They make a reasonable difference with the stock turbo, but they aren't the best choice for big turbos. Overall, it's decent quality and works ok.
I've heard of people having issues with programming these. Ended up going through a few keys then just buying it from the dealer. I'd just get it from the dealer rather than go through several eBay pieces. Friend of mine got a new Passat key and it was $250 out the door...ouch. Don't lose your keys ;)
This is all the stuff I can think of for now. If you have questions about any other parts, feel free to ask me.