At 15 years the refrigerator is at or a little beyond its life expectectancy. I'd pass on this deal.
Even if the unit was free and all you had to do was pick it up my answer would be basically the same.

Probably not. No more than $100 at that age. $150 if you just love it for aesthetics.
Older machines are energy hogs and at 15 years you are flirting with fans and defrost timers wearing out. I would look for something in the 5-7 year old range at the $250 price point. Craigslist's got 'em everyday. Just keep your eyes peeled for the color you want, wait for one to pop up for 300 to 400, offer them 250 cash and take it home.

Refrigerators can last a long time.

I have a KA top freezer unit that's 12.5 years old, never any trouble. Been in my garage for the past 5 years for drinks/spare storage.

My grandmother has a Whirlpool top freezer bought in 1984, no repairs.

My neighbor had a Whirlpool top freezer unit bought in 1985 or 1986, replaced a few years ago ONLY because the door liner cracked. He tried to align the crack by flexing the door, it broke worse (NEVER do that with an aged/brittle door panel, he should have asked me first, LOL). He wanted to repair it, but the liner was NLA. He checked the old unit and the new (KA SxS) with a KillAWatt meter and found that while the old box did use more power, the difference is such that it'd take a LONG time to get a payback if the old unit had been replaced specifically for the reason of saving energy.

I've got an Amana in my utility room that is older than that and has never had a single problem. I plan to replace it next week with the energy star replacement rebate before it does have a problem, but I sure wouldn't try to sell it for $250. I'll be more than happy if they take it away for recycling and I get the additional $75 on the rebate. I don't think I could take money from anyone who wanted to use it though.

I have a 29 year old Whirlpool fridge that's still going strong in the basement but I wouldn't pay $250 for an old fridge. As long as your current fridge is still working, save your money until you can get a new fridge.