You can disclose your ideas, designs and product plan details in various ways to try to make them readily findable by patent examiners doing searches. One defensive publication site is IP.com. It is fee based. But the best way to have a high probability of a patent examiner finding your defensive publications when examining someone else's application is to actually file them as (full) patent applications requesting early publication. You can expressly abandon the application and never risk getting a patent yourself. Of course each of your applications will have filing fees (about $700) and early publication will cost $300 more.
If you do not make your disclosure "findable" enough a patent could be granted to someone else that shouldn't have been. It will be presumed valid and take work and money to try to neutralize it.
Also, if you have a breakthrough idea, many people will probably come with valuable improvements that are seen as non-obvious over your original concept. They will be be able to get solid patents on those improvements and, if people prefer the improved versions, your vision of enabling a completely open marketplace will be significantly diluted.
Clarification (simplified and therefore not 100% precise)
Anyone can file a patent application on anything but if they are not the actual inventor they shouldn't get a patent. And even if they did truly think of it themselves, they should not get a patent if the things they are claiming as their own are something that has been already done or written about. They also shouldn't get a patent if they are claiming things that are obvious based on what has already been done or written about.
But the patent examination process is imperfect and not everything similar that has been done before will come to light in the USPTO searching. (That is a prime motivation for the existence of this site.)
So, if you do not want a patent but also do not want your invention, and obvious variations of it, patented by anyone else you need to be as sure as you can that your contributions are not just "out there in the public domain" but written about in detail somewhere that the patent examiners will be looking. Even if your product is on the market and you have a web site that does not mean the examiner of someone else's application will stumble across it. If they don't, the other person could get a patent on exactly your thing or on a variation on your thing that is obvious in light of your work. This is all separate from the issue of the other person knowingly ripping you off or innocently indecently inventing. The patent exam action process has no real way to look into that.
Now someone has an issued patent and all patents are legally "born valid". That means the burden of proof that their patent was incorrectly granted is on you or whoever else is in the marketplace. Even though you don't want a patent you can still get caught up in the system.
That is all motivation for you to publish your ideas, designs and teachings in a way that a patent examiner is very, very likely to come across them in their searching. The beat way to do that is to actually file a patent application. After it is published by the USPTO it should be found by a future examiner, preventing your idea and obvious various of it from being patented by someone else. Since you do not actually want a patent you can abandon the application and it will remain in all the patent document databases as a published application but you will not have the costs of actually pursuing to the point of getting a patent.
However that does not prevent people from getting legit patents on non-obvious extensions, variations, improvements of your idea. That would not stop you from making and selling your original idea but could prevent you from evolving your product (that is why it is a "blocking" patent) in a way that responds to customer needs but is not technically obvious from your original.
To make a broad region of variations obvious you will want to not just ship your product but somewhere publish a document that covers as many variations as you ca think of now. Then to get a legit patent someone needs something non-obvious in light of your writings not just in light of the product itself.