All vendors will buy items (for example - loot) from players for standard rates of coin. However, not all items are saleable.
All vendors sell items to players: different vendor types sell different kinds of items. See Category:Vendor NPCs for more information.
From level 10: armour, weapons and tools all degrade with use, and require regular repair to avoid becoming unusable. All vendors provide a comprehensive repair service for all repairable items.
LOTRO stores which items a player sold to every vendor: allowing individual players to revisit the vendor and repurchase or buyback the item. The buyback transaction is cash-neutral: meaning the vendor makes no profit during the transaction.
Not every item sold to vendor is available for buyback: LOTRO will alert players selling non-buyback items to a vendor - asking for player-confirmation before completing the sale.