Just to add to what's already been said ,quarantine is a must. Also I try to avoid buying from private breeders unless, a I know a little about their background and b I'm looking for a specific breed that I may not be able to find in hatchery stock. But that's considering I already have a lot invested into chickens.Not to echo the others, but a solid quarantine procedure and set up is an absolute must if bringing new avians onto your property. With as much physical separation as possible, and also methods. I feed, water, inspect my known birds first, THEN go do the same for new additions, THEN go clean up. Food and water are kept separate as well. I'm fortunate in that I can keep more than 200' between houses, and about 50' between the quarantine run and the main pasture (electric fenced), I know that's impractical for most - but even then, if I were to accidentally bring fowl pox onto the property, the local mosquitos range far enough they could potentially transfer. and since my quarantine run is outside my electric fence, housing new birds there is a risk to them as well. (I've not strung up the wire yet to fix that, need more insulators, but not currently in the budget (or the plans - not adding external birds this year).
Its all about risk management.
What you can reasonably do to mitigate risk, you should do - understanding that there are no guarantees - how much risk you are willing to take depends upon your investments in your current birds and your discretionary budget. Those are decisions I can't make for you.