Wow, I hate to say it but the dealer may be correct. These cars have the very problematic CVT (crap) transmission. They were so bad that Nissan extended the warranty out to 120k miles for the 2010 and previous models. So yours is in that category.
If there is no metal or chunks in the pan you might get away with a flush and refill of the transmission fluid to gain some additional time, and it might make no difference.
There is a diy section on this forum that tells how to change the fluid yourself.
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If this was mine I would change the fluid , flush the cooler or replace it, if it’s not leaking it should be fine, and see what the transmission was like at that point. The Nissan NS2 fluid that the transmission requires is available off amazon but it’s still nearly $20 a guart so a do it yourself flush and fill is going to be over $100 but It MAY help, there are no guarantees.
If it helps and the shifting is good take it to a dealer and trade it in. You might at least get your money back in trade in value.
Just don’t do to someone else what the previous owner did to you.
You might also consider small claims court against the former owner, depending on the laws in your state, but that’s probably more trouble than it’s worth.