concert tickets : eBay Guides

I've read 2 guides on not buying tickets on ebay for concerts as ''they have scanners'' and ''they check your name against the original buyers of the tickets'', etc, and I am intruiged as to how this works. I've sold a ticket before and as far as I am aware, the buyer was never refused entry or had their name checked. There's so many people that are attending these gigs every night, that do they really have time to check every single name against every ticket? Unlikely. As for for scanning-what do they scan? The hologram? Because doesn't every ticket have that anyway?

So if anyone could shed some light on this that'd be great.

Cheers.