Regional versions exist for Canada (Canada-Wide Astronomy Buy and Sell) and the UK (U.K. Astronomy Buy & Sell). General Marketplaces:

The Complete Guide to Buying and Selling Used Telescopes Whether you are a beginner looking for your first set of optics or a seasoned astronomer clearing out space for a new rig, the used telescope market is a vibrant community that offers significant value. Unlike cars, telescopes are often kept in near-pristine condition by hobbyists, making the secondary market an excellent place to find high-end equipment at a fraction of the original price. 1. Where to Buy and Sell

A highly reputable, paid membership site ($15/year) that deters scammers and serves serious observers.