Telescopes are unusually good second hand purchases. The optics do not wear out, the market is full of instruments bought with enthusiasm and used twice, and prices are typically half of new for something functionally identical.
What to check before you pay
Look through the front at the primary mirror or objective: a dull grey mirror needs recoating, which costs real money. Work the focuser through its range and feel for grinding or slop. Check that the mount moves smoothly in both axes and that a computerised one powers up and completes an alignment. Everything else is cosmetic.
What is expensive to fix and what is not
Recoating a mirror and replacing a computerised mount's electronics are the two costs worth pricing before buying. Focusers, finders, eyepieces and collimation screws are cheap and available from the brands. A scratched tube or a faded label matters not at all.
Where to look
Astronomy club noticeboards and society sales, where instruments come with somebody who can demonstrate them. Specialist dealers' used sections, which cost more and are checked. General marketplaces, which have the lowest prices and the highest proportion of magnification-led toys, so the aperture is the thing to read rather than the description.
What holds value
Large dobsonians and quality refractors hold value well. Cheap department store instruments hold none at all, which is another reason not to buy them new. Eyepieces from known makers barely depreciate, so a used eyepiece from a reputable brand is one of the safest second hand purchases on this shelf.
Questions people ask about telescope
Is it safe to buy a used telescope?
Usually, yes. The optics rarely fail. Check the mirror coating, the focuser and, on a computerised mount, that it powers up and aligns.
Where can I buy used telescopes?
Astronomy club sales, specialist dealers' used sections and general marketplaces, in descending order of confidence and ascending order of bargain.
How much should a used telescope cost?
Roughly half of new for a well kept instrument from a known brand. Below that, check the mirror coating and the mount carefully.