For my quarter I paid the rancher directly for the hanging weight based on current markets and paid the locker plant for the processing.
If they are taking it home to cut it up themselves, you cannot assist. However, if they are taking it to an inspected plant, you can shoot it, gut it, pick it up with a loader tractor and put it in their truck.. Also I've got two trips to the slaughter barn. One to haul the animal there and another to pick it up. Then I might have to put the boxes of meat in a deep freeze at home if said buyer isn't ready to get it when he said he would. Its just kind of a pain in the ass and don't really make much money. I also used to fatten some animals for a couple of guys, they would come and I'd shoot them, they'd gut em and take em home to cut up themselves. That worked ok for awhile, then as the market started to come up and I asked for a higher price, they haggled on me to get them cheaper, I gave in and thought this is the last time I'm fattening a steer for anyone but myself.
I do know of two ranches that are fattening their own cattle, having them slaughtered at a federally inspected barn, then selling the meat direct to consumers and one is even selling to a couple of restuarants. I'm glad it works for them but I doubt they're making much. Hauling them animals in to be slaughtered and then back to pick up the meat, storing it on farm, then hauling it all over to sell it sounds like a heck of alot of work when my day is already filled up with chores, fixing, and all the other fun stuff that goes along with owning cattle.