There are instances where this is possible, but the person selling would need some kind of authorization to handle the sale:
Could be estate sale with Executor handling the sale.
Could be trustee for a trust.
Could be someone with power of attorney from the titled owner.
Could be sherriff selling a home at a local tax sale.
Best thing to do is check with the broker and also check with the title company or closing attorney. They will generally steer you in the right direction and also check to ensure the person selling the home has the right authorization documents. I would do this early in the process not at closing. If there is a problem then you'd want to avoid the transaction or have time to get it resolved and know it can be resolved.
Your profile does not state your role (Realtor, buyer, seller, etc.).
If you are a Realtor I would discuss this matter with your broker right away.
If you are involved in any aspect of this transaction I would suggest (not legal advice here) that you check with the escrow or title company that you are using to clarify the situation.
As a real estate agent, the agent is acting as an agent for the broker. The listing is technically listed with the broker. The broker is acting as agent for the seller. If the person who signs the listing agreement is not on title, then in order for him/her to represent the owner, written authorization is required. Just as the listing agreement authorizes the broker to act as the seller's agent, same goes for the person who is representing the seller to the broker.
A power of attorney, a trustee, successor trustee, and similar positions can have the legal right to convey (sell) property, but the parties to the sale must all have "notice" (proof...a copy of the paperwork authorizing them to do what they are doing.)
The individual should have the full power of attorney from the titled owner. Make sure you see the original, let your broker/manager take a look at it - and keep a copy in your files, with a copy of the ID of the individual that has the power of attorney. The Title Company will need this information, too.