Outdoor antique market held twice a month on Saturday and Sunday mornings. There doesn’t seem to be any regular pattern as to which two weekends (this is Naples), so check first with the tourist office, website, or Qui Napoli (which is the most likely to be accurate).
The market is spread out along both sides of the Viale Dohrn, closed to traffic for the occasion, which runs through the Chiara park by the seaside, just west of Castel d’Ovo. It’s an ideal location; lots of space for parking, hard standing (although no cover), accessible, close to public transport, and room for several hundred stalls. It was described in my guidebook as gigantic, but on the Saturday I visited there was only a handful of stalls, all furniture dealers. There was more action on the Sunday, about 60 stalls, but it still felt very slow. Many dealers were still stalling out at 8.30am; there were big gaps between the stalls, and hardly any customers. It had rained during the night, which might have discouraged some, but what dealer is discouraged from visiting a good market by a little rain?
This would be a good place to get unusual Neapolitan souvenirs. There was lots of majolica, Sorrento ware (wood inlay), presepi figures (from the famous Neapolitan nativity scenes), and masses of pictures, from garish views of Vesuvius and sunny beaches to Victorian genre paintings and allegories. There was also religious art of all kinds; big boxes full of ex votos, reliquaries, rosaries, candlesticks, and sentimental saints. The furniture was mainly of the heavily varnished kind, but there was also some unpolished rural and agricultural stuff, and also some ship equipment, heavily varnished and of doubtful age.
This is primarily a public (tourist) market, but not a complete rip-off. The prices I saw were comparable with those in trade markets elsewhere, and all, of course (particularly on a slow rainy Sunday), negotiable. But it wouldn’t be worth a special trip, from what I saw.