A standard merchant is what you find throughout most of Vana'diel. Their prices are based upon the level of reputation that you have (the higher your reputation, the lower the price). They will also buy nearly anything from you (unlike those picky Guild Merchants and Tenshodo Merchants...). Standard merchants tend to sell general items and aren't particularly specialized like the Guild Merchants, Tenshodo Merchants and Regional Merchants are. They sell everything from scrolls, armor, weapons, food, furniture, etc.
Their prices don't fluctuate with their supply and they don't get "sold out" of an item. They are open all hours of all days. However, the items that they have available may vary according to the conquest placement of the seller NPC's city, and whether you are a citizen of that city or not. Some items are only available to citizens of the city when it is in 1st place, so keep that in mind.
Many items can be sold to NPCs, at values greatly below the amount you can buy it for from the same merchant. Occasionally, you can sell items to an NPC for more gil than by selling it at the Auction House. In FFXIclopedia, the price an item can be sold (to a Standard Merchant) for can be found on the item's page, in the "Other Uses" section, under the heading "Resale Price". Also, merchants tend to offer better prices to you when you have a high reputation, in recognition of your services. The formula used to determine prices based on fame is a complex one; in general, the range between the lowest and highest starts at approximately 40% for items below 1000 gil median price, and reduces to approximately 20% for higher priced items. There may be exceptions to this rough formula.
An item's lowest gil resale price is about equal to it's Guild Points value (if it is one of the guild's daily requested items).