My answers?
Gasoline, any amounts, but one and five gallon containers for counting purposes.
Toilet paper
.22 caliber cartridges

What do all three of these things have in common?
Once used, they cannot be reused. Well, in the case of the .22 cartridge I am sure some enterprising person will figure a way...

But gasoline once used is gone forever. Same with toilet paper thankfully.
But in many cases, hauling a couple of rolls of Charmin down to the trading post or swap meet may not be handy, will there be a more liquid form of money?

For many in the preparedness community, it has always been considered that gold and silver would make a comeback for post-SHTF money. After all, precious metals have a real intrinsic value that any of us would relate too.
However, I think some may question whether or not a Silver Eagle is indeed what you say it is. That is why I suggest that "junk silver", pre-1964 U.S. silver coins such as dimes, quarters and halves have a more convinient value than gold or specialty silver coins.
For those just joining us, junk silver coins have 90% silver content and are easily recognizable as legal U.S. tender.
More to the point, small silver coins will have greater value in common trades say for a dozen eggs, a bag of roofing nails or a quart of honey. What's more, common coins are easily recognizable and in most cases, trusted as not being forgeries. WHo knows if that Silver Eagle or Kruggerand is really a foil coated fraud or not?

My advice is as always is to stock up on real everyday tangibles first. Then, and only after your preps are at a comfortable state, consider investing in some junk silver coins.