I have no information at all about this case, but the newspaper report, like so often in such cases, is most probably false in many facts. I do not know of any ammunition that explodes by itself, because of a normal, summertime (up to approx. 27 C) , temperature rise. Wet P is about the only stuff that comes to my mind, but that would need air access also and it would catch fire, but not explode. No newspaper has a munitions specalist on their payroll, so mostly newspaper reports are wrong in many details because of a complete munitions- technical ignorance of the writer. Also, in the last 25 years, truthfulness in reporting seems to have become unimportant, Bloody headlines is what the readership seems to want, so that's what they get, even if truthfulness goes down the drain. I do not visit fleamarkets often, but I have never seen live WW2 ammunition being sold there.
Bellifortis.