Hi Bluebell.
In January 2013 I was offered and gratefully accepted a second hand 6×8′ glass greenhouse, with a view to installing it on my recently acquired allotment. I dismantled it and erected in time to get tomatoes and aubergine in etc. Great.
From my experience, if you go to inspect a second hand glass house, check whether the panes have been afixed with mastik instead of the more modern ‘foam tape’ option. The glass on mine was the former and it turned a relatively straight forward job into a marathon during dismantling and then, even more so, the clean up – hours and hours of extra work!
Great now it’s up and in use, but if I ever opt to do it again, I’d avoid any that have mastiked panes!
I hope that helps you and good luck with your build.