If the ebay cable is legitimately 6 gauge, it'd be fine and a decent value. I'll echo Al's concerns though, as the car/stereo world has a somewhat looser set of standards when it comes to wire sizing and quality then FRC or industry. My understanding from the Ebay listing is that the wire has no size printed on it and does not come on a spool; proving its size might be a little difficult at an event.
An alternative is to go through a 'known' supplier, like McMaster. An equivalent cable might be
http://www.mcmaster.com/#6948k91/=wjrygy. It's more expensive at $1.75/ft, but you'll know it's the correct size and as advertised.