Not supporting the dealer, but most chickens when moved can get ill - all of mine generally come down with sniffles when they arrive and most/ some require Tylan.
It sounds like mycoplasma which will rear its head in times of stress and require treatment. If your chickens are on Tylaninjections then there is a 28 day withdrawal period only because the injectable form has not been tested on poultry- however if its the stuff you add to water there is no egg withdrawal as it is recommended for poultry (most vets that are used to working with chickens would use tyaln in the water as it is less stressful for the chickens and its easy to treat the whole flock- unless a chicken is really really poorly - eyes shut, tail down, not eating or drinking as stress can kill chickens too).
Mycoplasma can be transmitted by wild birds and is very common in backyard flocks. If they are Hybrid chickens they should bounce back pretty quickly- even if they are organic free range chickens- Defra only requires them to have 1sqm of space for producers to put this on the box so they would still have very close contact with other chickens.
Chickens will also go off lay in times of stress, they like to be settled to lay even if they are classed as POL that does not mean they are all laying I have had POL birds that have been layers from 17 weeks and some that have waited until they are 30 weeks - do you know how old your chickens are?
I have always found some high protein food helps my girls when they are ill - I either scramble and egg (cooked) or get them some live mealworms this really gives them a boost.
I would also look to worm them when they are better- you can buy Flubenvet in powder form and mix it in the food or marriages do a feed with it ready mixed in.
I have dealt with many dealers in my chicken keeping years and many vets - most vets have no clue about chickens and after spending £100's on my flock I have now found a great farm vet who has yrs of experience. But the fact the dealer would let you return them is a good sign. I really hope your girls get better soon and you start to enjoy the antics of being a chicken keeper.