It is Poland. And work is being sold on some forum. I think suing him would be too expensive, am i right ?
I think you are probably correct that suing the person who is selling your work would be too expensive.
You are not going to find an attorney willing to work on a contingency basis except under extraordinary circumstances which do not appear to be present here. Attorneys who handle international lawsuits are not only familiar with the country's laws but also their codes of civil procedure, which are often vastly different than the laws and rules of procedure in the U.S. Attorneys handling international lawsuits do not come cheap. For many reasons, international lawsuits are prohibitively expensive to pursue for most people.
The copyright laws in Poland are far different than those in the U.S. There are no statutory damages, for one thing. You could potentially be awarded damages of double or triple the amount the infringer would have paid to license your work for use, or, in the case of reproductions and sales of your work, double the probable value of the benefits or profits received by the infringer with the sale of your work. This would probably not cover your costs of filing suit in the court in the area of Poland where the infringer is infringing.
You can, however, demand by way of letter (more effective when sent by an attorney) that the infringer stop the infringement. You can demand to be compensated for the infringer's unlicensed use of your work or the unauthorized reproduction and sale of your work. You can pray that the infringer feels guilty enough to meet your demands.
For a look at Poland's copyright law, you can visit the World Intellectual Property Organization's website (www.wipo.int - if the following link does not work):
http://www.wipo.int/wipolex/en/text.jsp?file_id=129377. For information on what a copyright holder can request of his infringer, see Article 79.1.