Came across this site, very useful advice for collecting also some good stuff on approx values on other pages of the site.
Came across this site
http://www.retrogamingcollector.com/Buyers-Advice.html
really like these statements
Be careful with any item where the seller says "It was working last time I tried it, 20 years ago."
It may have worked then, but condensation can ruin electronics, and after that length of time, there's a very good chance it doesn't work now.
Ask the seller to test it, and if they refuse, avoid the item.
Be wary of any item where the seller says "Untested, but it should work".
There's a very good chance it won't work.
You could ask them to test it, and if they refuse, the item is best avoided.
Avoid any item that doesn't include the power supply, regardless of whether the seller says it works or not.
This is a common ploy to hide the fact that the machine doesn't work.