While I know asolutely nothing about what a Intes Micro 715DX should cost second hand...
I have had decent luck with using Astromart (and old ads) for price discovery...while you can't see exactly what price was "settled on" for a transaction you can get a decent range...
There is also a telescope "blue book" (the name of an old price guide for used cars in the US) the name and cost escape me at the moment but it can also be a good price discovery tool...Good Luck!
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