If by "for the freezer" you mean you will be buying grown animals and taking them directly to a processor (or butchering them immediately yourself, although I doubt you mean that with hogs as it's quite a job to do just one!), then all you need to do is avoid animals that look likely to have problems that would render them unfit for consumption.
However I have a feeling you mean you want to buy feeder pigs and grow them up TO eating size; and you seem to want to keep the sheep/goats.
In which case I would say, very very strongly, "if you have to ask what to look for, you should really not be buying at a mixed livestock auction".
In significant part that's where people dump their culls, and many (most?) of the culls are not there because their ears were 2" too short to make good show stock or things like that, they're there for PHYSICAL REASONS like diseases.
If you aren't really pretty well-experienced with a particular kind of animal already, buying at open auction is just a great big huge kick-me sign. (By open or mixed auctions, I mean "as opposed to breeders' auctions, containing only one species or sometimes even only one breed", which is a bit of a different situation but not what you seem to be talking about here)
You COULD get lucky and have nothing go wrong, but, the odds are against you and buying sick animals can cost you a buncha money and germs can linger for *years* in your soil.
Just sayin',
Pat
