Depends on the flea market. The only flea market I know of around me is a little iffy when it comes to their animals. I'd only get chicks who are 2 weeks, not older. Make sure they are;
-active but not flighty
-clear eyes and nose (no discharge)
-happily cheeping
-feathering in correctly (no bare spots)
-no wounds or injuries
Some flea markets might not be super clean or well monitored for animal/bird health (mine is Not) so judge the situation as you see it. If Anything seems off to you, just get young chicks. and I know you havent responded about it specially yet, please make sure to get a thermometer! especially if you get baby chicks and can't find any 2 week olds at the flea market or on craigslist.