Well, i am not a rich person, i am actually bellow the average of that degrees, but a 870 is not actually that expensive.
I am saying that, because you can be sure with an used gun. How the previous owner treated it? What did he shoot with it?
And a whole bunch of other stuff.
But if the offer for the used one is really good, just make sure you will do a full field strip before you buy, after determine the age of gun.
Closely check all parts, even use a magnify glass if need.
Also, you can take a dummy bullet to test its function.
By the way, dose the previous owner have not any paper that tells when hi bought the gun?
I mean, there should be papers mentioning the guns serial, right?