Mountain Bike for my girlfriend...

[Grog]

I realise it's not as simple as that, as it never is.
The problem I've got though, is that at this auction, I will likely not have access to t'internet to check potential value of items, so a broad understanding of the good brands and stuff would be very handy.
It's an auction, so there may well be some right good bargains...

Well the thing is that among the major bike-shop-bikes manufacturers (let's say: Trek, Giant, Kona, Specialized, Marin, DeVinci, RockyMountain, Jamis, Norco... there are probably a few others), as explained above, the bikes are roughly the same in terms of range of quality. They all have entry-level bikes all the way up to super-duper-high-quality bikes. They don't really make bikes with crappy parts (compared to bikes you'd find at Walmart and cie.) but they sure make some that are better than others.

The BIG difference is the geometry of the frames, which varies between manufacturers AND between models. There is also a significant difference in the various configurations: some will have this kind of wheel with these components, others will go for a different mix, some put a specific kind of handlebar that one may like, etc. That becomes more of an issue as one gets some experience with bikes and starts knowing what one likes. However the geometry and the size are VERY important. If one's bike doesn't fit, it will just stay in the garage, end of story.

So buying a bike at an auction is a risky move. Sure you might get a really good deal... but if the bike doesn't fit her and is painful and/or scary for her to ride, you'll have wasted your money.

Otherwise, if you really want to do it, and are adamant on getting a "deal", you'll need to become familiar with the main series of each manufacturer (some have numbers, others have names) and be ready to think on your feet on the day of the auction.

Sorry for rambling; there isn't a simple answer to that question.