What is a nice way to tell someone I am planning to sue them?

I know there is no 'nice' way, but here's the gist, hopefully someone can help me. Earlier this year, a friend of mine was going to a big convention where he thought some of my artwork would be of interest to the people going there. I sent him with about 12 paintings, the smallest was about $50 and the largest ones, which were several, about $400-$500 each. He told me only a few had sold, and he would send the others back. I waited, and waited, and now he is avoiding my phone calls and emails. I recently sent him an email and he said he would get to it that week, but this was about three weeks ago. He does this, he buys time and then never does anything. I'm sick of it, and am planning to sue. I want to let him know I am planning on it, that if he would just do the right thing and arrange for this art to get back to me, and ACTUALLY get it back to me, I won't. What is a nice way of telling him that? BTW, I have doentation of him taking the art, as well as several of these emails where he says he will, but never follows through with it. There is also a magazine article he wrote that states the art was taken by him to this event, so I think I'll be good in court as far as evidence goes.