Calibration can be as simple as having a signal source with a known (and accurate) frequency source and amplitude, and using that to confirm that the horizontal timebase and vertical amplifiers are accurate.
*real* calibration is a little more complex and a lot more expensive.
"needs calibration" can mean anything from
"This is a perfectly good scope that is fully functional and accurate, but the calibration certificate has expired a week ago"
to
"Following an incident, the timebases are all completely wrong and the vertical accuracy is shot, I haven't looked to see if there is any damage, so I'll just say it needs calibration"
As a beginner, it will be hard for you to know the difference, and may (in the worst case) result in you effectively wasting your money.
Look at eBay this way: Sure you can get stuff cheap, and that is money saved. Every time you save some money, you can think of that as capital to emotionally offset a purchase where you do your dough.
I've bought lots on eBay and I am so far ahead on buying stuff that I can justify spending a several hundred dollars on something described as "for parts or not working", and another hundred on freight, knowing that if it didn't work I couldn't return it. If it didn't work, I'd be able to shrug and console myself that I was still ahead. (But I still wouldn't risk it for something anywhere near $1000).
You probably can't do that. The money you're planning on spending is probably a lot for you, and you're probably not in a comfortable position of having already got so many bargains that this one matters less.
1) Look for auctions where they offer some form of return guarantee
2) Understand how to test the equipment before you buy it
3) be prepared to test it when it arrives.
And also:
4) Ask the buyer how well it works if there are not pictures of the actual item in use.
5) Be patient.
6) look at the sellers feedback profile and the nature of the other things they sell.
7) If it's an auction (as opposed to "buy it now") decide how much you're prepared to spend and bit that amount as close as possible to the end of the auction (within the last 5 seconds is ideal). Don't chase it (and bidding at the last second pretty much means you can't).
8) look for people selling lots of scopes. Chances are they know how to test them (if in doubt, ask)
You could also sign up to a local freecycle group and ask if anyone has a scope they're throwing out.
If there's a hacker-space nearby you could join that. Apart from perhaps getting some more hands on help and experience (my local one runs things like "arduino nights" teaching people how to use microcontrollers for example) you might find someone there who can help locate an oscilloscope or help you test it. They may even have oscilloscopes for communal use; if they are within easy travelling distance, maybe you don't need to buy your own just yet.
