80-400mm question

I am somewhat interested in buying this lens for focal length offered, and FX compatibility if I decide to for FX body next year.

I am being offered a 10yr old example of this lens at 1/2 of its today's sticker price.

Considering I can get 20% discount at local camera stores for new gear, do you think its a good deal?

--
Rick Halle wrote:

" Keep in mind that tall buildings sway back and forth so they require faster shutter speeds."