The length of patent validity. Each patent has a certain period of validity, such as 10 years for utility and appearance, and 20 years for invention. When purchasing, you need to pay attention to how long the patent is valid.
Patent holder. Check whether the patentee is a patent seller and whether there are multiple patentees. Avoid the situation that the seller has no right to sell the patent and the contract is invalid.
Patent type. There will be many similar patent names. Make sure that the type of patent you buy is consistent with the contract.
The current authorized use of the patent. Whether the patent has been licensed for transfer or even exclusive license, all these need to be listed in the contract to avoid purchasing the patent with exclusive license and being unable to use it.
Whether the patent is in the right state. (whether the annual fee is not paid in time or declared invalid, etc.). )