Not everyone you cross paths with will be worse off than you. In fact, unless you are an extremely well off prepper, you're bound to find people who don't necessarily need you. Those folks you really might want to barter with, but you might have a hard time convincing them.
As SilentBob said, I understand what "Not enough ammo" is. I've never been in a situation where I thought I'd taken too much ammo. I understand that there is the issue of not being able to load it out, and that's a different story. If you can't carry it, then... You can't carry it. If you can get ahead of the curve, like Henley said, then you can get ammo to where you intend to BO to.
This idea of splitting up pistol and rifle ammo really opens the discussion up. You may live an area where a 22-250 is "Too much" gun. If you're taking shots 90% of the time, that are within 50 or 75 yds, then you really may want more pistol ammo. Depending on where you live, a 22-250 may not suit you're needs as well as say, an AR or an AK. Then too you come across the weight of the 45. The weight of 45 vs 9mm just doesn't play out well IMO. I would rather have 9 due to it's availability and it's weight, which when talking about thousands of rounds, can save quite a bit. My $.02