"Yes, my friend. Step into my shop." This is the phrase you will hear as you approach a kiosk in Nairobi or in any other marketplace in Kenya. I love buying things in Kenya. It's a mini-adventure. The prices in a marketplace are generally not fixed unless it's food or there is a sign indicating the prices are fixed. It is expected that you will try to bargain the prices down.
The salesmen in Kenya are very good. One man approached me with a pair of wooden giraffes. His price was 800 shillings - about $12 US at the time. Not a bad price when thinking in US terms. I really liked those giraffe and decided I wanted them. I started talking with the man about the price. I usually low-ball the price. I told him I'd give him 100 shillings. He laughed and told me that he'd give them to me for 600 shillings. I told him 150 shillings. Eventually we settled on 300 shillings - about $4.50.
One trick I found was if I walked away from an item - even if I REALLY wanted it - I could get a better price. If the salesman can sell the item for the price you are asking, he will come after you and agree to it. If he can't, he won't follow you.
Once I got burned on a belt I wanted. I settled on a price with a lady on it, but I walked away and came back about 45 minutes later expecting that price. She wouldn't sell it to me for that price, but for 100 shillings more. Darn. I bought the belt anyway. It was my mistake and I didn't repeat it.
Sometimes you can trade an item, like a piece of clothing (usually a t-shirt), to lower the cash price of something. I didn't do that. First off, if you were to add up the cost of the item you are trading plus the cash you are handing over, your total cost is usually higher. Secondly, since I was living in Kenya and not a tourist, I needed my clothing. If you are a tourist and are making room in your luggage for all those curios you bought - by all means trade your clothing.
Don't be shy about bargaining. The vendors are not going to sell their wares at a loss. Decide on a price that you would like to pay and then try to bargain towards that price. Be patient. You may spend ten or fifteen minutes bargaining on one item. Laugh, smile, be friendly, and have a good time.