I have a Swarovski Z6i 2x12x50 BT that I'm thinking of selling to a buddy of mine, but not sure what's a fair price for both parties? The scope is 2 years old, in great condition and was purchased for $3300. It was on a Sako 85 Finnlight that I sold and the gun had only been shot 20x. I was planning on putting the scope on a new rifle that I was going to buy, but have since decided to go another direction on a custom long range rifle/scope.

Thank in advance for your help

Natchez Shooters Supply had that model as demo scope they were selling $2500. http://www.natchezss.com/z6i-2-12x50-bt-4a-i.html
Since your's has been mounted and fired, I think you would be looking lower in price.

I sold a used in box Z6I 5-30x50 for $2300

I sold the same scope, illuminated but without the BT (had the discontinued TDS reticle which is really hard to find) for $1750 a few months back. You might get a little more than $2000 on eBay but will pay them 10% to do it.