Beginners help

[fedemail]

Hello everyone, it may be unusual but i am in a weird situation and maybe this forum could have some advice for me.
I just bought a collection of old calculators and pcs. this includes a dozen of HP (9810a, 9830a, 9845a and b, 9100b etc) and some other brands like olivetti, wang, singer, friden and various gadgets, plotters and spare parts. Problem is, i have no knowledge at all to test/try them. I think i can handle the olivettis and the handheld ones but surely not the 1970s pc machines. These items will be for sale later but i will have to test them to describe them at best.
heres my questions, first of all, they surely have been stored and unused for years, is it safe to turn them on or it may cause damages? Is it possible for a total neophite to quickly understand how to turn them on and test the basic functions? also, some of them have monitors installed, some have not, do they need specific ones? should i just sell them as untested or the working/not working value difference is too much to ignore?
Sorry if this is out of place in here but i'm in Italy and these machines were not that much common here, i found nobody able to help me and i am trying all i can.
Thanks in advance and sorry again if this is in the wrong place.