Hats, shoes, and robes
An item's buy and sell price are influenced by its color. A black base with black trim is the most expensive color combination, while the cheapest varies by gender. Items with a brown base and brown trim are the least expensive for male wizards; if your wizard is female, items with a white base and brown trim cost the least.
The same applies for changing the color of any of these items. "Repainting" an item black on black will cost more than changing it to any other color combination.
The buy and sell prices of reagents bought at a vendor do not change. (Typically, the buy price is about the same as what is available through the Bazaar.)
Selling to a reagent vendor, however, will always get you the lowest possible price. In the case of widely available reagents such Cat Tail, there is little difference; you'll get 3 gold from a vendor and 3-5 gold at the Bazaar. For rarer reagents, however, the difference can be significant. Reagent prices at the Bazaar are based on supply. If there are only a handful (or zero) of a reagent, its price -- buy or sell -- will be much higher. For example, a vendor will give you 3-5 gold for a piece of scrap iron, but you can get 40-50 gold at the Bazaar.
Similarly, when buying at the Bazaar the price of reagents will fluctuate from transaction to transaction as the supply changes. For example, if there are three Diamond for sale, the first might cost 200 gold (supply of three), the second 300 gold, and the third 450 gold (supply of one). Ditto for selling.