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Iain
With dobs the obvious ones are:
1) Cleanliness and condition of mirrors (both primary and secondary
2) Condition of the mount, often these are made of chipboard so easily affected by moisture. The base board particularly can be affected as it is out of sight and on the ground)
3) Condition of the focuser, if it is gummed up with dirt it may not be useable
4) Finderscope ie is it still there and is it in good condition.
5) Collimation adjustment screws on both mirrors. If rust has got in, they may not be useable
They are the obvious ones. Best way to avoid is be patient until one comes up that you can inspect.
Malcolm
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