Buying them from an auction makes quarantine even more important. They will have been exposed to everything that any other bird there is carrying. That is a very scary thing. Made my jaw drop. WOW um ok so how to quarantine when you cannot get them far enough away....
Before you buy them...
Check the legs and feet for signs of scaly leg mite, bumblefoot, abnormalities etc
I do not know if they will let you but ask if you can check them or at least one of them for mites/lice
Look them over and watch for the signs of illness. Eyes clear and bright, alert, feather condition, breathing normally not wheezing etc.
Then if you do buy them perhaps do the following....
1. Can you keep them in the garage for a month? ( In a walled of section of course not just running all over)
2. Do not wear the same shoes when tending them and then the original flock.
3. Tend your existing chickens then change shoes and clothes then tend the new chickens.
4. Wash hands every time you get near them, handle the water or food dishes, preform an exam etc
5. Treat them for lice/ mites since that is something they are likely to be carrying
6. Have a plan on how you will deal with any that show signs of illness. Are you prepared to end the suffering of one or all of them if they present signs of being ill?
If you do not quarantine by separating them properly you could infect you existing flock and need to be prepared to deal with the results. It sounds harsh but in reality it is a must to be able to make the tough decisions sometimes.
Having them sharing the fence line is not quarantining them. The board or plywood is not adequate. It is just a physical contact barrier not a germ/ disease barrier. Sorry.