When you can offer unrelated gauranteed pairs that is the ultimate.
But that should also demand the highest price. I would gladly pay more for this gaurantee if I was a serious breeder. If I just wanted birds that may possibly breed or just as pets then the extra $ wouldn't make sense. So I guess it all depends on your target crowd that you are trying to sell to.
It would be nice if you could keep them long enough to visually sex or by song. This is what I am doing with my parrot finches. Pretty owls you have there
I have received birds that have been shipped. Actually most birds we buy from the pet stores or from bird brokers at the fairs have been shipped in boxes. Yep, they are. These boxes I have picked up from the airport have been varied. One poorly constructed with exposed staples that the birds could have been hurt on, one that was the type that poultry would be shipped in, and one super nice kennel (like what a dog would be shipped in) modified for birds with smaller screen and perches. I have never shipped and do not plan on it. Though I feel that it is not a problem via the airlines. All my birds arrived alive and in great shape.