It's quite simple.
Bike manufacturer orders 5000 groupsets, uses 3000 for production & sells remainder on. That deal would be set up in advance of the delivery of parts, so the surplus goods barely stop before being passed on.
Recipient of the 2000 groups pays below regular wholesale for the groups, so can sell for a big discount off retail without taking a big hit on their profit.
Looks like a winning formula all round, everyone's happy. They're making money & you get a deal. Until you need the warranty.............
Then you're stuffed, because the ebay guy can't service the warranty, because the buck stops with him. Sram, Campag, Shimano won't deal with him because they know that they sold those parts to the manufacturer, who sold them on the quiet. No chain of supply, no warranty trail.
Warranty is the last refuge of the RRP retailer.
I jest of course, but there is more than a grain of truth in that.
It's easy to overestimate the value of a warranty on most items - I often see this with people expecting a bigger than reasonable discount because an item is being sold secondhand. Ignoring completely the facts of how often they actually claim on warranties.
Why take my word for it when a whole industry of selling warranties on top of products has grown up precisely because people rarely claim on them.
Almost every product I've ever bought has worked flawlessly for a year. And I'd be quids in big time if I could have had 10% off the price I paid and no warranties on anything.
It's not worthless ... but many people way overvalue it.
Stiff, Light, Aero - Pick Three!!