iMac 2011

A 2011 iMac is obsolete, you cannot run any OS more modern than High Sierra. That means that many apps you may want to run will be unable to be installed.

If you do buy, it's best to have hands on experience with it before buying. Many folks have been burned by buying used Macs online. Be sure the previous owner has followed these instructions:

What to do before you sell, give away, or trade in your Mac - Apple Support

Also it would be best if the previous owner has the install DVD's it shipped with because you may not be able to use Recovery without them.

Not from Apple. Apple hasn’t sold those for many years.

You can certainly buy one on ebay or the like.

Can you clarify your second question?