I think you are absolutely NUTS buying a used camera lense.
Unless you can hold it in your hands and inspect it yourself, how do you know if there are any scratches/abnormalities?
If you are buying used, get it locally so you can see it for yourself.
So much can go wrong even if you were to buy it at a reputable forum.
I live on REDUSER.NET and buy frequently there but lenses at a low cost are tricky.
If you were to buy a CP.2 or RED PRIME (costing above 3K for most) then you have the buying power to return the item since most users there are pretty good for that kind of thing.
But again, Id hate to be shipping gear back and forth.
So costly
I've bought and sold ~20 high value lenses on different forums over the last few years and have never been disappointed or had any complaints. You deal with reputable people that buy and sell a lot and take good product shots.
It also depends on the lens you want to buy. Most good lenses hold their value very well. So if buying a $2000 lens and can only get a used one for $1800 or so, it makes sense to just buy new.
I just got a Canon 85 f/1.2L for $1650 that was barely used and when used, only in a studio. That's a decent savings for a $2200 lens so I sprung that route and couldn't be happier. I found a listing mistake on a new 35L from a local shop that was under cost and got that for less than many were selling for used. You have to look, find the deals and go for it. They are all over.
Plus, you sound like you have no idea what you want. You want either a prime or a zoom? Well duh. And then you are choosing between a zeiss and tokina? Why those two? I loved my tokinas but they aren't in the same league with a zeiss. Hell, if you want a cheap fast prime, go with an old nikon or something with an adapter, you're manual focusing anyway. Look at the Rokinon primes. I loved my 85 f/1.4