This is the place every tourist gets sent to by any guide or tourist book to buy Ghanaian 'things': wooden masks, carvings, fertility dolls, kente cloth, beads, brass figurines. The name is something of a misnomer. This is more a craft market than an artist's haven (those can be found in the more wealthy shopping malls). It also has its ups and downs as a place to visit.
On the up side, just about any curio you can buy anywhere in Ghana, you can get here - and at about the same price; which saves you from having to lug cloth, statues and masks all over the country while you travel.
On the downside, its huge and aggressive. One of the more pleasant things about purchasing goods in Africa generally is sitting quietly with a local seller (who may even have made the piece him or herself) and chatting while slowly negotiating an acceptable price for the thing you want. You're not going to get that at the National Arts Centre. The people there (as with most people in Accra) are there to make a buck.
Its hard work to get a good deal, and not a particularly pleasant experience, but you can find what you want and, with a little persistence, get it at a price you like.
Look out particularly for fertility figurines, which come in a great variety of shapes and sizes, and kente cloth - there is a great variety of designs and sizes (from one strip to nine strip), and the cloth is well-priced. As for the back rooms where all the "genuine antiques" are stored, and which you'll be shown with a great deal of conspiratorial gesturing and circumspection, well I take those with a pinch of salt.