I’ve done a lot of buying and selling on Gunbroker.com. Would suggest looking there.
If you aren’t familiar with the process of buying a gun online, it’s actually very straightforward (as long as you’re not in a state like CA). The transaction basically just has to flow through a local firearms dealer — that is, someone who carries a federal firearms license (FFL). You buy the gun online, then you let the seller know which FFL to ship to. You then have your local FFL send a copy of their license to the seller and they will get it shipped. Once the gun arrives at your local FFL, they will enter it on their books and contact you to come pick it up. You then fill out the transfer form and it’s yours.
Most local gun dealers will perform transfers for a small fee, usually around $25.
I’m a fan of the Mossberg 500 and Maverick 88’s as well. I find the position of the action release more natural than an 870. Completely personal preference though. Both are excellent guns, tried and true.
As far as specifics like barrel length, I’m a fan of a simple 18.5” or 20” cylinder bore (ie. no choke) if it’s just going to be a closet gun. But if you intend to hunt or go shoot clay birds as well, you can often find “combo” guns which come with a cylinder bore barrel and a field barrel that can be interchanged.
EDIT: One more thing I’ll add. Don’t be bashful about buying a used Mossberg 500 or Remington 870, as long as there is no visible sign of misuse or rust. Home defense shotguns are one of those things that many people buy, but few people actually shoot. As a result, many of the used ones that get sold have almost never been fired. Then the day finally comes when someone wants some cash and decides they don’t need the gun anymore. As long as they are stored in a reasonably non-humid area, they can sit around for years with virtually no maintenance and be perfectly functional.
In short, there are some types of guns I would be very reluctant to buy used unless I had an opportunity to inspect it very well. Remington 870’s and Mossberg are not those types of guns. No sign of rust or misuse? Good to go. Hope this helps.