Be vigilant when purchasing items as many cheap things, that you are offered, are just that: cheap! You may be told that an item is made from unbreakable stone, just to find that the item showed to you was made from a different material from the one you bought, or the fantastic papyrus painting has faded and crumpled on your journey home. Like many tourist destinations, Egypt has its fair share of vendors who prey on unsuspecting tourists and will scam them when the opportunity occurs. If you are going to purchase items, try to make sure you go to a reputable outlet: papyrus museums for papyrus as on street vendors tend to sell items not made from genuine papyrus; market stalls in large markets (like the Khan El Khalili) for trinkets; clothing shops for genuine cotton clothing.
If you are with a guide, get him/her to negotiate a camel or horse ride for you as you will have less chance of being cheated. If you do not have a guide, try and find one who will do this on your behalf. This is especially true on the Giza Plateau.
Try to avoid purchasing anything off anyone selling items in the street, or at tourist site car-parks. Also be careful if someone approaches you and tries to get you to follow them to a shop that has items you are interested in. These shops are usually down a side street and will scam you.
If you feel that someone is cheating you, or being too aggressive, contact a policeman as soon as you can. They are plentiful and one of their duties is to protect you.