If you buy your used conservatory from a private home then the person who dismantles it and re installs it is as important as the quality of the conservatory you buy itself. Ask the installer for pictures of previous work they have done on similar conservatories and always ask for written references that are check able.
You should plan a visit to at least two previous installations and see how the home owner describes the installation time costs and any problems that may have arisen. The plan for installing your conservatory should be typed with deliverable like installation date and what is involved.
The most important thing you should do is make sure the installer of your conservatory conforms to all regulations and is registered with people in the fenestration trade body and have a registered business address even if they are self employed they should at the minimum have a registered name.