although DM has some mobility problems and is quite frail, it still works for them. They’d only leave if something happened to one of them.
“Of course we’d pay full market value but it would be good to avoid those annoying estate agents fees”
I'm a solicitor and I would be slightly wary of this because the world is full of people trying to get bargains regarding housing by playing on people's frailty or vulnerability. It goes without saying that any sale of any house in these circumstances should be at market value, not a bonus someone would give you for avoiding estate agents. At its worst, this could be undue influence and I would advise your parents to tread warily even if they were considering this offer, and to get independent legal advice.
Of course what you are really doing by avoiding estate agents when selling a property is avoiding putting it on the open market, and that leaves a seller open to all sorts of potential under-valuation or money off for "necessary repairs", etc. Everyone knows that a surveyor commissioned to act for a buyer can be influenced into putting a lower value on a property by talking about supposed concerns in advance.
Perhaps if the note had mentioned an independent valuation the interested parties would have paid for, or a premium above market value, it might have come across better, or even an offer to pay your parents' legal fees so they could be independently represented. But the inference has to be that those people are somehow doing your parents a favour by paying market value.
Not that your parents want to sell anyway! But I think your concern is warranted and people should be very careful in situations like this. I would also not respond and not engage with them in any way. If they put another note through the door I would then reply asking them to stop contacting your parents.
I've got a house on the market myself which is proving slow to sell and the number of times I've been contacted by companies or an individual trying to get it for less than market value, along with all their persuasive arguments about how they would be "saving me money" and "helping me" are quite shocking. One even phoned me this week and started arguing with me and talking over me on the phone. And I'm not elderly.
Market value is what the open market will pay, not a closed, restricted market decided by one party. You and your parents are right to be sceptical. Healthy scepticism does no-one any harm.