A peddler is a specific type of salesperson: someone who travels from town to town selling their wares.
A peddler is someone who sells things, but it's a very specific type of selling. Peddlers — also known as hawkers and pitchmen — travel from town to town, especially with a carnival or circus. Peddlers are also found on the street, selling many different things, from jewelry to DVDs. There's a shady implication to this word, since peddlers have included people selling snake oil and other worthless products. Instead of buying from a peddler, you're probably better off at a store.