Concierge.com's insider take:
Each Sunday morning, the streets just outside the ancient Porta Portese gate fill with stalls and bargain hunters in Rome's biggest flea market. There's a lot of rubbish here—"designer" T-shirts and "Louis Vuitton" bags straight from the Far East—and the wily dealers who run the antiques stalls in Via Ippolito Nievo always get the price they want. But rummage in secondhand book stalls or among boxes of vinyl LPs, and you may find some gems. For clothes, new and vintage, the scrum of choice is in Via Sannio, just outside the walls by the Basilica of San Giovanni Laterano. Each weekday morning, and all day Saturday, push through the cheap tat out front to stalls in the dark interior for fascinating finds. Pickpockets love the crush at both, so be careful with your wallet.