Telescopes are sold through four channels that differ far more in what happens after the sale than in price, and the used market for them is unusually good because the optics do not wear out.
New, from a dealer or the brand
A specialist dealer will tell you when the instrument you asked for is the wrong one, which on a first purchase is worth more than any discount. Buying direct from the brand is the simplest way to compare printed prices, and five of the eight brands in this index publish one in dollars on their own store.
Used, and where to look
Astronomy club and society sales come with somebody who can demonstrate the instrument, which makes them the safest. Specialist dealers' used sections cost more and are checked. General marketplaces have the lowest prices and the highest proportion of magnification-led toys, so read the aperture rather than the description.
What to check before paying for a used instrument
Look through the front at the mirror or objective: a dull grey coating needs recoating and that costs real money. Work the focuser through its travel. Check a computerised mount powers up and completes an alignment. Everything else that goes wrong is cheap to fix or cosmetic.
Selling one
The same channels in reverse, and the ones that reach astronomers get better prices than general marketplaces do. Clean nothing except dust, photograph the optics honestly, state the aperture and focal length plainly, and include whatever eyepieces and accessories came with it, since those carry real value of their own.
Questions people ask about telescope
Where is the best place to buy a telescope?
A specialist astronomy dealer for a first purchase. Direct from the brand for the simplest price comparison, and club sales for the safest used buys.
Where can I buy a used telescope?
Astronomy club sales, specialist dealers' used sections and general marketplaces, in descending order of confidence and ascending order of bargain.
How do I sell a telescope?
Through channels that reach astronomers rather than general marketplaces. State the aperture and focal length, photograph the optics honestly and include the accessories.