I wouldn't be afraid to use eBay at all. I did the timing belt on my A4 with parts from eBay 4 years ago. Never had a problem and it had 186k miles when I wrecked it (had about 120k when I changed the belt). I recently took off the heads to use on another project and the belt looked like new. Just check and see what brand the parts actually are. Mine was a belt made by Continental (tire/rubber company). I believe the kit was like $125 with the water pump?...that could be wrong..

+1 on the blauparts kit.

There are two different 2.8L engines, the older 12 valve and the newer 30 valve engines. In 1998, the A6 sedan had the 30 valve, while the wagon had the 12 valve.
So, be sure to get the correct timing belt kit, tools, and instructions.