Take the serial number and plug it in at Starlink.com/Activate. If you can proceed to checkout, it's not active and you can take ownership of the dish. If it says already assigned, it means the current owner hasn't released it from their account, and you won't be able to do anything with it until they do.
There aren't really any moving parts, it generally either works or doesn't work. Where you are buying it from? If you can meet in person, I would ask to power it up and make sure it will connect. Even an inactive dish will connect to the satellites, download updates, and have a "Restricted" status in the app. If the dish won't connect you can pass.
Depends on where you live. In many places in the US the Standard dish is on sale for $149. So $100 used isn't that great if you live in an area with the discount. The MSRP is $349, so $100 for a working kit is good.
Ask the seller when they purchased the kit. They come with 12 months of warranty coverage that transfers to the new owner.
