This particular collector goes by the handle Flak88. If he's not a Neo-Nazi, or at least a Nazi fetishist, why would he use Neo-Nazi code for "Heil Hitler" in his username?
Maybe he's just pretending to be a Nazi so he can get good deals on militaria?
I'm sure he means the 88mm Flugzeugabwehr-Kanone (FLAK), the notorious anti-aircraft and anti-tank cannon of WWII.
My dad and his captain had a VERY close encounter with one on the national highway near Piacenza during the German retreat. Fortunately the shell passed harmlessly between them and exploded on down the road, but it startled them to say the least.