ha, found my last similar post from a year ago:
I have snapped* precisely 2 carbon seatposts
1 – A genuine USE Alien in the early 00’s
3 – A genuine Bontrager jobbie, last week
*also cracked an Easton one but removed before the chance of bum splinters got too high.
Neither were over tightened or crashed or subject to any kind of excessive force, although both broke slightly above where they were clamped.
I have also got a couple of eBay carbon posts that have not broken (yet) despite one of them having been battered into the ground quite hard during an accidental ‘exit through the front door to river’ incident during a race last month.
As always, strength and longevity will be down to individual build quality, which may or may not be better on a branded part, *hopefully* decent testing and standards are followed whether generic or banded, but shoddy ones will slip through in both circumstances, but maybe more prevalent in cheaper options, especially if obvious ‘fake’ items made to look/stickered up as genuine, vs decent but own brand/unbranded.
In short, make your own mind up
There are however plenty of decent Alu posts at decent weights, and decent carbon posts in sales at major retailers so probably a better bet there than random chance on eBay.
What’s your reason for wanting carbon, is it weight or ride feel?
Weight wise it’s only really top end stuff that is any lighter than the decent Alu options, and ride feel is very much model dependant as some are quite stiff and others much more compliant so some research may be required if you’re after something specific.