I'm sorry. It sounds like you've had a couple of tough years. Maybe I can set your mind at ease about what your Mom paid for her first machine and the trade in amount she was given. If her first longarm was a full size longarm with an 8, 10, or 12 foot table, yes, those can easily cost $15,000 new from a dealer. $8000 is about what an owner would get for a used set-up (machine and table) selling it on their own. $2000 for trading the machine in on the Tiara does sound a bit low, however, it would depend on the age and condition; the used inventory the dealer has; and is it the same brand that the dealer specializes in. It's just like a used car in those regards. Look at it this way, too -- the Tiara I think is around $6000. If the dealer gave your mom $4000 or $5000 on the trade, they'd be just about giving her a brand new longarm for a used one, and that wasn't going to happen.
I think you'll get a fair price for the Tiara selling directly to another quilter. Take care, and, again, I'm sorry for your lose. It is never easy to loose a parent. No matter how old you are, it takes you right back to being a child and wanting Mom or Dad because they're the only ones who can make it all better.