Answer to the question:
- as there is a patent and you believe it to be valid, it is illegal to use the patented invention.
- it is totally irrelevant if you make any money on the invention or not, the use is exclusively for the patent holder (or licenced users).
- but, well, the bad consequences will occur if the patent holder finds out and actually cares.
I believe there are five parts, or maybe phases, to the question about patents. The steps are mostly consistent around the world although the details may vary.
- Getting a patent
There is a process where a person or a company may ask for and be given the exclusive rights to a patentable "invention" for a specific territory and a specific time. It includes describing what is to be patented and paying the relevant fees. The patent authority, granting the patent, does a bit of research including trying to check for patentability and finding "prior art", that is if this is already patented or perhaps generally known through a publication. In different countries this checking may be more or less thorough, I believe that the US patent office is known to be less strict than in some other countries. Once the patent is granted it is published. (Sometimes companies avoid creating patents as the secrets are then published).
- Using the patented "invention"
The idea is that the patent holder has the exclusive right to use the patented invention for a limited time. This is only relevant for the territory where the patent is granted, as example, a US Patent might not automatically be valid in France. No person, may use the patented invention without permission of the patent holder. There are a few exceptions to this, but the infringing party then is generally governments or international organizations (examples include some medicines, or the way AIS data is sent between ships).
- Stopping patent infringement
The patent holder will try to stop others from using the patented invention. This might start with a "stop and cease order" with wordings warning of dire consequences. Often the idea is to be frightening enough to not have to go to court. It might escalate to going to court and asking for damages. It is generally up to the patent holder to initiate this action, although at times authorities or border forces or such might do it on their own. Border forces are sometimes instructed to look for "counterfeit" brand products punishing the consumer buying these products.
- Going to court
The typical situation is that there will be less probability for a court proceeding the less money the defendant has. Sometimes in the US law system the civil layers will take cases on a percentage of the eventual penalties (this is not allowed in some jurisdictions), one reason why they aim for astronomical sums. But if the defendant has no money, the "ambulance chasers" will not take the case.
- Trying to disprove the patent
It is not uncommon for the patent to be proven "not valid" in the court process. If the defendant can show "prior art", the patent was invalid from the start. If it can be shown that the patent in a specific case was not relevant for the actual usage it will also be rejected by the court.
So, you might want to take your chances. Maybe the patent can be proven to be non-valid if you can find prior art? If not, it will be illegal, and could become costly.