Everquest currency has four types: EQ platinum, EQ gold, EQ silver and copper. And EQ platinum is the most valuable and copper is the least. One silver is equal to 10 copper. Also 1 EQ gold would equal 10 silver or 100 copper. All NPC vendors will accept any type of coin as long as it is the equal of what they are asking. Players have a lot of influence over the economy in EverQuest. Sometimes a certain high level item costs quadruple Everquest Platinum or half in only a single day. Because of this prices are always changing, especially as changes are made to certain items and new items are introduced into the pool of trade.
But you can always find out about the NPC vendors making EQ plat is the strategy sell high and buy low. You will find a strong player economy in EverQuest with a demand for goods from all levels of play. there are two places are great for buying or selling the good.
The first one is the tunnel in the East Commonlands. You can easily get to this area from either Freeport or Neriak. Also, many other cities are close enough that it is not a cumbersome journey to make. The other place is the Bazaar which you can find on the moon of Luclin. You can always find players are buying and selling a variety of goods and services in these two places.
There are always trades happen in any semi-populated zone, especially those zones that happen to contain a bank. But for the most variety, and sometimes the highest sell prices, you will need to visit one of these two zones.
When you have enough Everquest platinum and in need of something specific, you can go these places. There are some changes on different servers throughout the world of Norrath which sometimes promote one of these locations over another, and sometimes even a different location. Especially, North Freeport, people are selling in any zone with a bank. So generally this is often server specific. You can buy some specific at these places.