A aluminium frame is lighter then a steel frame. Old bikes and retro bikes often still have steel though. You can recognize an aluminium frame by its thicker tubes and welds. You will most probably not find carbon and titanium frames at such an auction.
Determine what kind of gears you want.
In case of an internal gear hub: Look which models and brands of gears would suite you. For example: for a gearhub you might want to have 7 of 8 gears from Shimano, or perhaps 3 gears of any brand would be sufficient.
In case of a derailleur: The most common brand is Shimano. I have listed them for you from bad to good: Altus, Acera, Alivio, Deore, Deore LX, Deore SLX, Deore XT, Deore XTS. For mountainbiking you'll need at least Deore but rather even better. This shifts quicker and lasts longer in case of mountainbiking. For commuting you want Deore and up, or Alivio if you wan't a real bargain. These are sufficient for commuting since less wear occurs and quick shifting is less important then with mountianbiking.
Altus and Acera are less reliable and don't shift as comfortable, so avoid them. Also, when a cheap Altus or Acera derailleur is assembled, other parts are often also of less quality.
Other parts such as steer, cranks, saddle post, etc. can be from a well known brand such as Shimano, SRAM, Contec, AVID, Sturmey Archer, and many more. Or, they can be no-names.
I know this is all not very precise, but it can give you a clue about the quality.
(edit: I hope I improved my answer a bit)