Inevitably a trip to Nepal will involve some level of shopping. I look on this as an opportunity to get my XMAS shopping done and/or a chance to replenish the "emergency gift drawer".
If you are in Nepal for a trekking holiday, I would recommend that you book an extra day at the end for your shopping butt if this is not possible then check with your hotel in Kathmandu as many will check luggage while you go up-mountain. This means that you can get your shopping done and check it for the period of your trek. At this point I'd also recommend that you check a nice clean outfit to change into once you return to civilisation and want to get decent again.
My shopping experiences in Kathmandu centre around the Thamel district where most trekkers will tend to stay. Whether you are shopping for gems or pashminas the best thing to do is get up bright and early and be the first person at the shop in the morning. There is tradition/superstition among shopkeepers to make that first sale of the day so they will be particularly amenable to haggling. The last thing they want id for the first customer of the day to walk out empty handed as this will set the tone for the rest of their day.
Gemstones - I shan't tell you my favourite place to buy stones but I will say that it reminds me of the little shop in Chinatown at the beginning of Gremlins. In general the thing to do is to sit down, have a chat, have a cup of tea and peruse the goods at leisure. Get a good look at all of the things that you are interested in before focussing in on what you want to buy. Get prices for everything that you are interested in and then try to get a bulk buy price. I usually go for a price 25% below what they ask but if I think they're being cheeky then I aim for 50%. In Nepal I focus on buying Star Ruby Sapphires and Watermelon Tourmalines. General rules for buying gemstones apply, size, clarity, cut should all be considered.
Pashminas - Prices vary wildly due to the variety of levels of quality. Quality is affected by percentage of wool to mixer and whether the mixer is cotton or silk as well as where on the goat the wool has been gathered from. Underside of the goat is better moving from belly (lowest quality of the underside) to under the chin (best quality). Again, I settle on a level of quality, agree a price, then see what price I can get for a bulk purchase.
Yak wool shawls - are a cheap and lovely alternative to pashminas but they do tend to shed initially. That said, the huge price difference is compelling. I pay no more than £5/shawl even in Namche. A must-buy for the cost conscious.
Wool - a lovely alternative to buying a finished product. Buy the wool (raw or dyed) and make your own souvenir!
Spices - I prefer the raw spice selection in Marrakech but if you like Tibetan or Indian food then it's worth investing in some of the ready-made spice mixes. Frankly they are so cheap that it's hardly worth haggling over these. Just be sure to check dates in the packages and buy the freshest.
Clothing - It's worth considering getting a Shalwar Kameez made. What you do is buy the fabric and pattern then get measured up and pay for a seamstress/tailor to make the clothes while you are away trekking. I confess that I haven't done this myself because I hadn't realised last year that one cannot buy these off the rack. Also I was there for Teej so most of the selection was in red. :-(
Funny story there - during Teej, married women fast and pray for a happy marriage and unmarried women fast and pray for a good husband. One flirty guy asked why I wasn't wearing red and off at the festivities (with waggling eyebrows). I told him it's because I'm a widow. That shut him up!
Carvings and Brasses - All look like cheap tat to me so I don't bother with them.
Antiques - be careful here as exporting anything over 100 years old is forbidden. If in doubt then get a certification.
Any other questions about shopping in Nepal, please post in the comments. And don't forget to donate to The National Museum of Computing to support their fine work to preserve our computing heritage!