Hey, I actually like the idea of it being in a type of once per month thing. Like Coltzan, except, well once per month and with a LOT better prizes
The neomail issue is hard to say. Because you have to remember that ever second, TNT are giving out so many neomails (regarding auctions, warnings, etc) that if they increased this amount, would it make a massive difference (site performance wise)? A shop would be complete chaos. Too much lag I reckon, and there'd still be a way to sift through and find the best items, whilst the worst items will stay to rot (eg, if there are 500 of a certain type of item, and 1 type of item, which will you most likely get? And then think about the fact that if the shop has a max capacity of 50 different types of items, it'll be clogged up with different variants of junk.).
Random Events will be too dependant on refreshing as everyone seems to have pointed out to me. So having a type of 'once-per-month' location to visit, with random prizes would really work. But maybe making it once per week? You have to remember that not everyone will be visiting it every month. I dunno.
The only question I have, it what if they do in fact run out of items? Should they give away random piles of Sludge or just close it?