Let's say, you file a provisional application in Jan 2018 but you have not filed the non-provisional within one year i.e. Before Jan 2019
Another person filed a non-provisional patent of the same invention on Feb 2019 and the patent get granted in Dec 2020.
Are you allowed to sell the invention since you can produce certified copy of the provisional application in court to prove that you invented the system before person B?
Not in general. Under the law in effect on those dates, the U.S. (as is the rest of the world) under a first-to-file system.
If you published your invention prior to the priority date of the issued patent you could attack the validity of the patent as not novel or obvious in light of your work. Importantly, if you made and sold the item before the priority date, that it is also prior art. The law says prior art includes -"or in public use, on sale, or otherwise available to the public before the effective filing date of the claimed invention".
There is a narrow provision § 273. Defense to infringement based on prior commercial use in the law that could possibly apply. If you used your invention to make a product that you sold but the process or machine used in manufacturing itself was not "available to the public" then, if you meet specific criteria you can keep making the product where you have been making it in the quantities you have been making it in.
