Sorry to say, but UN stamps sell for only a small fraction of face value (often 25% or less) mint, and hardly anything used. Even plate/imprint blocks sell for very little.
The exceptions are the 10th anniversary souvenir sheet (both types retail around $20), a few errors (imperfs and color errors, like the Prussian blue from the first airmail set), and the UN administration of Kosovo (which were never sold to the philatelic market). If you have any of those, they are worth some money.
EDIT to add: also the Flags issues, in full sheets, often still sell at a premium to face value. Your Flags blocks might be worth face or a little more if you can find a UN collector that needs them.