I concur with muchatrucha, it is an issue if it has been altered. Any alteration in Chamber dimensions or forcing cone does take the gun out of proof and should be disclosed as it affects value.
That said, depending on the buyer they may not care if the gun is altered or in fact might prefer it as it may be an alteration they would do if not already done (typically many buyers prefer the chambers lengthened to 2 3/4, but not all)
The effect of any alteration on safety of the gun in terms of blowing up is an individual question that has to be measured and looked at on a gun to gun basis. While Ed maybe correct that he has not gotten complaints, that is not the same as proving the alterations may not have on a couple of cases actually made the gun a question mark for how safe it is. He is correct that in the USA it is up to the individual shooter to determine, but that said most shooters do not possess wall measurement equipment or the experience and judgment that is required for the call so they are in some respects at the mercy of the seller.
I would not hesitate to by an altered gun, given the seller had a return policy as I feel comfortable measuring the gun and determining if I feel safe about shooting it and with how heavy a load.
Blanket statements on safety are not a good way to go, as there is more to the question and shooters must learn and understand the variables and what level of risk they are willing to accept.
If you don't have the knowledge, tools, and experience; then go to a doublegun knowledgeable smith (note not every gunsmith, if not most gunsmiths, understand these issues and has the correct measurement tools)
