Time to have that chat. I doubt your birds caught something at their new home in 3 weeks time. Possible, but unlikely. I believe he might have sold you some sick birds.
Chickens don't get colds. What they get are respiratory infections, which are usually highly contagious and dangerous (and often render a bird a carrier for life should they survive the initial illness). I would ask him why he feels they have a "cold"? What is he seeing to make this...