Yes, it's essential to prevent the new bird bringing disease(s) to your flock. At least 2 weeks, and longer if you have any doubts.
This is what I'd do in your position
1. Ask why the seller is getting rid of it.
2. Look for clean bum, feet, eyes, nose, and check there's no sneezing, wheezing or other respiratory issues with it.
3. Red comb and wattles.
4. Watch it walk, eat, drink.
5. If anything gives me cause for concern, I don't take it. There are lots of other birds out there, and it's much easier to prevent disease than to treat it.