As the title. Where can I sell my patent ? Is there a free website ? I searched around, but nothing found.
I'm not sure you're asking the right question here. No, I don't know of any websites for sharing and licensing patents, but that's probably with pretty good reason.
I think you might be oversimplifying the problem here. Patents are amongst the most advanced and technical parts of the law. Very few people disagree with that statement, in my experience. There are many nuances how patents are used, and what sorts of contracts need to be drawn to support them. It would be difficult to accurately capture the use-cases of every licensing deal through a free website.
In reality, at a potentially more relatable level, you'd be highly unlikely to get a good deal for your patent on such a website. To license IP, people have to know about it, know about you, and want it. Just as companies like Microsoft probably don't buy their server equipment on Craigslist, it's unlikely that the most lucrative of your potential licensees would happen across your posting on such a site.
Websites like that thrive off the volume of mass-marketing, and that's decidedly not how the patent system works.
Your only viable bet would be to sell to someone who then sold to a larger company, and aside from the inherent risk in such a position for the middle-man (a risk some would take on, no doubt), you'd likely be at a loss.
The best option in any legal situation in generally to speak with an attorney, and I'd suggest you do that in your situation. It's possible that an attorney would suggest to you, "I know this great free website for licensing your patents," but that's fairly unlikely. With an attorney who feels confident enough in your patent's value, you might be able to strike a deal based on equity if money is a concern, that would give you a good review of any contracts, and perhaps some hand-holding through the licensing and presenting process.