Keep in mind that if these are old bikes they will most likely all be steel. Many will be lugged. You might keep and eye out for any lighter weight lugged steel road bikes from the 70's 80's, especially the lugged ones. French, Canadian, or Italian ones are good bets. Raleighs can be nice to ride if in good shape. Most bikes have decals saying where they were manufactured.
Some old lugged steel "road bikes" might have nice headlights and fenders- these may actually be very nice older touring bikes and might be worth more. I'd stay away from the heavy Schwinn type boardwalk cruisers as they are so heavy to ship and their larger metal parts tend to rust.
There are afficionados out there looking for those old lugged road and touring bikes.
Snap up anything marked Waterford, Bridgestone, Mariposa, if they are only superficial rust....and if you see anything named Rene Hearse, RUN, don't walk, with it to your car.

I'm sure others will mention other valuable brands to watch for. My brain is fuzzy right now.