I recently built up a bike and bought items from these 3 ebay retailers: ediscountbike, buildyourbike and eurobikeparts.

Had no issues with any of them but one of the rear brakes from eurobikeparts had a problem with the spring. I emailed them and received a response the same day and a tracking number for the replacement part that night. Item arrived a few days later with a prepaid return postage label. That's pretty good customer service from an ebay retailer IMO.