After completing a main story quest, you're teleported back to Sanctuary into a room that is full of chests and other lootable boxes. The obvious thing to do is to take it all. Apparently, this room is Marcus's weapons warehouse and a door leads out into his shop. The door locks behind you and you're not able to get back into it from that side. Is it possible to enter this room again and rob Marcus of more delicious loot?
A character can actually be there twice, but sadly only by getting there in the normal playthrough and the "True Vault Hunter" mode playthrough.
Apart from that there is nothing in my personal experience and on the internet suggesting an alternate entrance. The door is locked and there is no quick travel point. Close examination of Marcus entire store yields no success.
It would be amazing considering the amount of gun boxes in there.
So sorry to disappoint, but should anyone discover anything post it immediately!^^
Possible solution to this is while playing multiplayer mode, one player must be on that current mission that leads you to Marcus' storage room.
Once in the storage room , loot all you can and trade to the player(s) who isn't hosting that mission. The player(s) with the loot walks out the door while the host stays in the room. That player can then save the loot in their backpack/storage whatever and save/quit. The host player quits without saving thus not finishing that mission. Rinse/Repeat.
Note * Probably best for Host to save and quit before you get to ANGEL core. You will have to repeat the mission's boss fight.