to underestimate the true weight of the bees you are selling.
I have found that if you hook up a very strong vacuum to an airtight box of bees (I prefer to use the air intake of my truck while it is in motion for the best power), you will get the best results. Make sure you put a filter at the start of the hose, otherwise you suck in all the bees. That was an expensive couple of mistakes.
Also, the bees sometimes explode. I don't know how to combat this. But if you are lucky, you pull out enough air so the bees wings don't have the ability to lift them off the ground, enabling you to get a true reading of the weight of the bee colony you are about to sell.
Good luck and let me know if you want more beekeeping business tips.