Silk is a great buy in Bangkok, but remember that there are a variety of qualities available. Hand woven Thai silk, perhaps the country's best known craft, is an excellent buy, available by the length, as tailor-made outfits, or in a huge variety of accessories and decorative items.
Make sure you are getting a good price on something you buy - Thailand is the land of the really good copy. Although salespeople may tell you that a piece is very old or authentic, don't buy it unless you know for sure. Be aware that some of the gold sold as 24-karat in Bangkok's "thieves market" is somewhat less than pure, but it is likely to be well crafted. Those Rolex watches sold for as little as a few dollars on street corners are, of course, counterfeits. The same thing goes for those US$4 designer clothing.
Trendy clothing is available at reasonable prices in the major Bangkok department stores, such as Central and Robinson.(The sizes, however, are very small compared to some standards, so watch out.) Also shop for lacquerware, celadon pottery, painted umbrellas, tribal weavings and handicrafts, temple rubbings, custom shoes and clothing, bronze, silver, gems (if you know what you're doing), opium weights and pillows, temple bells, sand paintings and carved water buffalo bells. Bargaining is common in markets and with street vendors, but not in shops.