Kathmandu is a genuinely good place to buy certain things. Pashmina shawls and scarves are available at much better prices than anywhere in the West; buy from shops in Patan or established stores rather than street sellers, and feel the weight and texture — real pashmina has a particular softness. Thangka paintings (Buddhist scroll paintings) range from mass-produced tourist pieces to genuine works of serious craftsmanship; good quality thangkas cost real money and take months to paint. Thamel has dozens of shops selling trekking gear — much of it North Face, Marmot, or Patagonia knock-offs at very low prices, though quality varies.
The Asan market in the old city is the place for traditional Nepali goods: spices, dried fruit, household items, and ritual objects. New Road has electronics and general shopping. Indra Chowk is good for fabrics and Tibetan goods. Avoid buying antiques or items that might be classified as religious artefacts — export restrictions apply and customs can confiscate items.