It's a dilemma isn't it, considering the forum celebrates whistle 'tweaking' and 'tweakers'.Altering especially the high end whistles that command top prices ought not be allowed without original instrument maker's permission, otherwise, the instrument is no longer that top ender. Buyer beware, and questioning whether instrument has been altered in any way ought to be asked with every purchase.
I would agree it's only good manners to make a prospective buyer of a used whistle aware alterations have been made. I have seen some really ugly examples in the realm of uilleann piping of people doing things to perfectly fine instruments they just didn't understand, including makers taking their tools to instruments of other makers and essentially ruining them. So I would be wary of that sort of thing and I have in the past responded to similar discussions on the flute forum with a degree of vehemence. Respect for a maker's output, knowledge and intention is a good starting point. But whistle tweaking is usually put forward here as almost a necessity and certainly something looked at with a degree of esteem and people are often encouraged 'to have a go'. So where do you want to draw the line?
I must say, I have had a greater percentage of perceived 'high end' whistles (let's face it the only high end about them is often found in the price) that didn't live up to their promise and proved a disappointment. Mind you, I am not a great buyer of whistles so the sample size may be limited compared to some here but I do play them.
FWIW, most used whistles I bought through the forum looked like they had never been played much at all so I am not so sure it's a real problem to begin with.