You want to be beware of the circling sharks who want your listing. The mental discipline of a private seller is important, especially given that agents are masters at undermining the confidence of sellers so as to convince them to list with their agency.
Real estate agents are specifically trained in ways to do this, and it's no coincidence when an agent phones you about your listing and says things like "we have people ready to buy right now" or "we just sold the property up the road so you should list with us".
Real estate agents even rehearse specific scripts that are designed to break down a private seller's resistance and to destroy their confidence in their own ability to sell privately. Protect yourself by watching some of the dodgy tactics agents use on YouTube so that you know what to expect.
A quick search on YouTube will quickly bring up dozens of videos of agents speaking to FSBOs (For Sale By Owners) in a sneaky and systematic way designed to get the listing by convincing the seller that they need the agent.
In Australia, some agents are told to scour the papers for private seller's ads, contact them, undermine their confidence and sign them up to an exclusive agency with vendor paid advertising.
If an agent insists they have a buyer for your listing, consider granting them a 1 day open or sole agency agreement where you specifically retain the right to sell privately. This would allow them to show that one buyer through while ensuring they only receive a commission if this particular person buys. Make sure you also negotiate the commission to reflect the fact that the agent will not be incurring the usual costs and has not had to market your property themselves. Think critically and be ready to walk away.
If an agent contacts you promising a buyer, ask yourself why this supposedly keen buyer wouldn't simply contact you through your existing listing.