Hopefully the days of pin values going up after you buy them are already gone, but if they aren't they'll be here soon enough.
I've flirted with selling my TNA, not because I don't like it, but because I've flirted with selling every game I own, pinball isn't really doing a lot for me at the moment. I'd expect to lose money on most of them. That's the normal way depreciation works. Maybe I'd make it out okay, I have nice examples of in demand titles, but this notion that things can't go anywhere but up is just silly, and it's well past time stop even entertaining it.
If you pay $10k for a TNA you're an idiot, plain and simple.