Easy crowns/heat
As you know looping is the normal process of going solo, clearing a level, and then because you don't go down at the end of the level you are able to rejoin the level and clear it again.
One small variant on this is to clear a level without picking up any crowns/heat after each kill. Simply leave them on the ground. When you get to the end of the level simply invite your alt to anchor the level, go solo, run back through collecting all the heat/crowns on the ground, rejoin your alt, and repeat.
Items, hearts, and potions are only able to be picked up once, but all Heat, Crowns, and Minerals can be looped and duplicated. Combine this with inviting 3 alts at the beginning of clearing and you can easily make 4-8k/40CE without having to clear the level more than once.
Granted this is a pain to setup, but once you do its SUPER easy money and fast.
Yeh misread the op, completely missed the mention of "items" expiring, seems like way too much hassle to be worth it to me then
Yep guns are the easiest. It's really not that hard on a GY level.
Arenas are okay....you have to herd the mobs into the center and kill them there so you still have room to move around the edges. The problem with Arena's is the party buttons only allow you to do this with one arena and the boxes afterwards. I have gotten some arena's that were worth probably 800-1000 crowns per level, but GY is much easier.
You can do this on any level really, obviously 2nd strata levels are the only ones worth it, but considering you can loop the level in less than 5 mins to make 1-2k crowns per person on the level...that's a good 20-40k/hr...with 3 alts on the level too you're looking at 80-160k Crowns/hour + all the heat you could want.
Doesn't seem worth it?

Exploit must be fixed! Hah fun sucker headed your way! This is almost like the time when I was a kid I exploited Neopets and found a glitch that got me over a million points...

Orly? I never had very many neopoints... I won a paintbrush somewhere and traded it for tons of items tho. Ah, the old days...
I thought crowns and heat weren't carried over for people who Go Solo or get kicked.
Why would you exploit Neopets? The pets were so cute D:
Anyway, back to the OP, yea that is a real bad thing...worse than Arena looping imo because you don't actually do anything D:
Guess you did this gun only? Also, did you manage it in an arena? Can't imagine that being very doable unless you're somehow able to herd all the mobs into one corner then kill them from out of reach
interesting in any case, but sounds far too frustrating to me (mats would be simple as they dont have any magnetism to your character however, and i can see this being worthwhile if you noticed multiples of valuable mats)