We buy quite a few bull calves in the 4-6 wt range at yards. It does not take that much in a band, a few minutes time through the chute, and vaccinations. We figure somewhere in the neighborhood of maybe $20 / head ... that is still less than buying steers of the same age/weight/quality.
If I had a neighbor that would direct sell me a group of bull calves, for around the going bull calf price at the sale, and do the work myself, I would jump on it. They do not have the exposure to disease by coming directly off the farm to my place.
The ones from the sale barns have more risk for diseases, but at that weight and weaned, are less susceptible than buying younger ones pulled right off a cow.
Most feeder sales have certain requirements for the calves to meet anyway. Most of ours here have a 30-45 day weaning, at least one round of certain vaccines, things like that.
I would buy at a feeder calf sale and keep them up for 1-2 weeks where you see them everyday, make sure they are eating, , no respiratory issues, and then go on with them.
My preference would to be to buy the neighbors bull calves if presented with all the choices...work them and keep them in the 2 weeks for observation, and go from there.
By the way, we use the Callicrate bander on everything of any size, they ALL get a tetanus shot as well as blackleg on the first trip through the chute... we wait for 1-2 weeks to do more vaccinations since their immune systems are stressed just with the banding and moving to a new place... then find that we have much better luck with the vaccinations after that. PLUS, we give an A&D vit shot , for help with the immune system for pinkeye resistance, or Multimin to help boost the body's ability to function.