How often do big dobs come on te second hand market?

[Astralstroll]

Are second hand dobs with a decent sized mirror (~16 inch) a bit of a rarity?

Dave.

[Qualia]

Probably more so than a 150 or 200, but I do notice them cropping up from time to time 16".

[Moonshane]

with the telescope house sale I'd buy new currently.

as Rob says, they come up less frequently than smaller scopes (sales like a pyramid with larger at the top, smaller at the bottom) but if you are patient you'll get a good one. If you are good at DIY then making your own truss dob around a good mirror set is do-able too. mirror sets come up as regularly as full scopes. anything above 16" is really quite scarce as a scope and more common (but still not common) as a mirror.