Woody, I have purchased several transmissions " off the floor" where I couldn't test them and have been very lucky.
If the transmission is being sold as in excellent condition, I would not purchase it unless it came with a written money back guarantee.
A transmission is not that difficult to rebuild if you have the book on the unit. You will probably end up with a better transmission because you will install better quality parts than your regular transmission shop, and getting the unit done at a specialty shop will be expensive.
I would suggest that you use a synthetic fluid for your tranny, whether you rebuild or not. It will shift smoother and run cooler. I have been using Amsoil fluids for 28 years, and have always had excellent results.
Bob


55 Cameo 396 TH 400 with overdrive
65 Caliente conv
67 BB Caliente conv
67 Mercury 1 Ton
76 GMC 3/4 ton 454ci
84 Buick Turbo Regal