A fellow local plated tank enthusiast offered me 115 lb of black and red flourite, not mixed, for $60. I estimated that it is around $150 in flourite. So am I right in understanding that flourite is mostly inert and there is no problem in buying used flourite. If anything it is already washed, which I here can be a task in itself.

Should be no problem at all

Reusing the flourite is fine, just verify with the previous owner if he/she uses osmocote/osmocote plus tabs; it could be a potential problem.

What problems can osmocote tabs cause in flourite because I was thinking about using them myself?

Flourite begins inert, and then binds nutrient cations until plants need them released. If anything, a slightly use flourite should be a better product for your plants, assuming it was used appropriately.

If it was exposed to an unusually lot of aluminum (not useful to plants) or something like that, then much of the CEC receptors would be filled and it would be less useful.

Using osmocote tabs is fine with flourite as long as you keep them in the gravel bed. If previous owner had osmocote, you might want to clean them out since they could be anywhere when you reuse gravel that have osmocote in them.

I would not hesitate to reuse it. The idea of problems with Osmocote might wash out as they float to the top when you rinse it a bit? Used Flourite might actually be better than new as all that fine stuff is far more likely to be gone than when you open a new bag that has been shipped and thrown around making lots of dust??? I rinse stuff and judge from the first rinse how much loose gunk I need to remove. Some starts clean , some stuff not so much. I would rather rinse it out outside and avoid waiting for days for the tank to clear.