If the seller offers to provide an extended warranty, make sure that it is an official AppleCare+ extended warranty which you would then need to confirm through Apple (the Apple website may not show the information in a timely manner). Confirming the extended coverage may be tricky since it may take days or weeks for Apple's systems to be fully updated with the new information. I don't know if it is possible for the seller to then officially transfer the coverage to you so you are listed with Apple as the official owner. As long as the warranty information appears on the Apple site, then it should be covered, but in some regions I believe the owner may also need to provide valid proof of purchase in order to verify the warranty coverage. If your region requires valid proof of ownership, then the proof of purchase from a third party gray market reseller probably will not be sufficient to confirm ownership (this is also needed in order to unlock a Mac which has been disabled through FindMy or other means). It is very risky to purchase used Macs, or unofficial third party gray market resellers for these reasons. Best to only purchase directly from Apple or Apple Authorized resellers.
Do not accept any third party warranty coverage, because those plans usually involve using a repair shop of the warranty provider's choice which may not be Apple authorized. Even if they are Apple authorized, you may only be able to deal with that specific repair shop which may involve a shipping the laptop to the repair shop. I find that many vendors providing third party warranty service will do a poor job, will take their time with the repair since it involves a third party warranty contract, etc. because they want to service their regular clients first which are the client which keep them in business with repeat business. Many times these vendors which do third party warranty work only get a minimum amount of money from the warranty company to cover labor fees.
Like the others have mentioned, if you are unhappy with the six month remaining warranty, then return the laptop for a full refund and purchase a new laptop directly from Apple or an official Apple Authorized reseller.