The main thing to watch is the condition of the primary mirror.
Depending on how well it has been cared for.
The mirror coating may be deteriorating or damaged in some way.
I believe it is not uncommon for mirrors to need re-coating after about 10 years.
Another killer can be mold if allowed to remain damp, or kept in a damp environment.
Ask questions like how and where it has been stored.
Has the mirror been cleaned. If so, how?
Check other glass surfaces similarly (secondary mirror; finder scope; eyepieces too).
Make sure rocker box is in good condition. (MDF and moisture don't mix well). Assess the effect of any DIY mods.
A preloved scope can be a bargain, with extras, accessories, and beneficial mods already made, but remember, you (generally) don't get any warranty, and you need to feel comfortable that this risk is covered by any cost saving.
BTW Welcome to IIS
