rebuilt transmission that has been sitting

I'd say yes, buy it. At $200.00 you can't go wrong. Just make sure it's not a lockup style transmission and is going to fit your application.
After sitting for 5 years, the seals might have hardened and need to be replaced. If I was in your shoes, I'd take it apart to make sure it has been rebuilt, and if nothing else, replace all seals internally and externally.
Just my opinion, others may vary.