Estate agents do it all the time so why not you !
I ignore those ones too - but at least I assume that, were I to respond, the EA would attempt to find the best offer from their (claimed) pool of interested people and advertise it on the open market.
I did this years ago for a specific house in a village but it turned out that the person who lived there had been diagnosed with a terminal illness. Our approach caused distress.
To be honest, although it would be the very last thing you'd want or intend to do, you never can know how it might be received. My family member had bad MH issues and, had she been physically well enough in her later years to see and deal with her own post, a letter like this would have really distressed her too. Whereas a less vulnerable person would see it as a 'just in case' free-to-ignore expression of interest, she would have remembered the name of the person who signed the letter, been hugely disturbed by it and shouted it up on a daily basis in perpetuity to the voices in her head about 'this Sally Smith who's trying to get me out of my home so that she can live here instead'.
How can you possibly know the top price that your house could fetch on the open market? Of course, everybody would prefer to avoid the EA fees if they could, but if you end up selling for £20K less than they could have got you, you've not really 'saved' by avoiding their 1% or whatever, have you?
You get that they’re not asking you for a free or discounted house, right?
Maybe it's just me, but I would assume that they were hoping to bag themselves a good deal - and thus have me sell it for significantly less than I could otherwise have got for it on the open market. What other possible reason could they have for contacting you directly other than to secure the purchase before anybody else has a chance to offer you more?
Kirstie and Phil always try to use their fame/muscle and push for offers made through them to include a demand to also take it off the market. Now, a lot of buyers use that as a tactic to strengthen their hand, but how does that actually benefit the seller in any way - especially if they do it the instant it comes on to the market? Others may disagree, but it seems highly, highly dubious to me. I see it as a clear admission that they know I could get more for it from another buyer - if they were confident that they'd offered the best market price, they'd be more than happy to wait a week or so for that fact to become evident when nobody else offers as much/more.
In fact, if somebody put a note through saying "could you kindly let me know if you ever decide to put it on the market" (as opposed to "let's do a private deal ourselves"), you can still avoid the EA fees if you end up selling to them, having tested the rest of the market. When engaging the EA, you declare that 'Sally Smith' has already approached you personally and expressed a keen interest in buying. Then, if the EA finds another buyer offering a better price or preferable circumstances, you can go with them and pay their commission; if however, you end up choosing to sell to Sally anyway, you don't need to pay the EA anything, as they weren't the ones to introduce Sally to you as a buyer.