Not an EA but bought and sold 8 houses over the years and been subject to both the G’s (though not done it myself).
You’ll get a lot of people on MN taking the moral high ground in both actions but I’m pretty philosophical about it.
A house is (for the vast majority) their primary asset and most expensive purchase/sale and my experience is that the majority of people will take actions that serve their personal financial best interests, morals be damned when they are faced with such a situation in real life.
Not everyone of course, but most.
if you choose to do this then you will need to contact the EA with your offer or the vendor directly (for example I had an offer posted through my letterbox).
Is it all about the money/offer? No. Just (if not more important I’d how proceedable you are.
In my case £10k more from someone who still had a property to sell was not worth risking the firm lower offer from a committed buyer (who had behaved impeccably through the process) with no chain.
Other people may have felt differently however depending on the urgency to move and the relative value of that £10k to the house houseprice vs the risk.
If the offer had been £50k more may have given it some thought rather than dismissing it. Similar at £10k if my buyer had been flaky/delaying/trying to re- negotiate etc
In short it can (and does) work if you have a very good proceedable offer and the chances of success are higher if the current bidder is messing the seller around.