It isn't worth a dime until you see it. "Adult owned" means zip. Seen a lot of adults ride like kids and nothing says the adult isn't letting the kids ride it either.
Nice Triumphs often command top dollar. Less than fantastic condition and their values go into the toilet.
The bike has been ridden only to keep the battery charged? Means lots of short trips. Would have been better to store it properly than ride it once a month for a few minutes to keep the battery ok.
I'd ask for the once a year oil change receipts, battery receipts (can't be the original, if it is, new battery time).
Fine tooth comb time. Tires changed for time or worn? If worn, how about the brakes?
It could be a real jewel or something that needs brake lines (were the brakes bled on schedule?) hoses (was rad flushed?). All that stuff.
Send the buyer the laundry list questions. No answer, walk away, they made a bunch of them.