How to buy an expired patent. Once you've located a patent that has expired, you can contact the patent owner and negotiate a sale. You can buy the invention and all rights to it, including the patent. You then renew the patent by paying the lapsed fees.
You can definitely sell someone else's patented product in their country of registration if they themselves legally allow you to do the same, or if you are able to take their approval to sell their patented product, conditionally or unconditionally.
Technically, it is possible to patent a new use of an existing product. Realistically, it might not be worth applying because you would have to show that the new use is nonobvious. Even if you believe your use to spectacularly nonobvious, expect an uphill battle.
Since the essence of the right granted by a patent is the right to exclude others from commercial exploitation of the invention, the patent holder is the only one who may make, use, or sell the invention. Others may do so only with the authorization of the patent holder.
If you or your business was targeted by a patent troll, and you received an assertion of patent infringement that you feel was sent in bad faith, please file a complaint with the Attorney General's Consumer Protection Division. Information about filing complaints to the Attorney General can be found here.
While the value of old patents may vary, there are still opportunities to monetize them. Whether through licensing, selling, or pursuing legal action, inventors and companies can explore different avenues to make the most of their intellectual property.