In re: buck meaning a marker for “you are it.” In the old days of paper correspondence and no copiers, offices would circulate items among staff as bundles. Accompanying each bundle was the “buck slip” which served both as a means of routing the set of papers–letter, article, whatever–to the right individuals and also a means of determining who had seen it, since before passing on the item one needed to sign or initial the buck slip.
Link: What’s the origin of the word "buck"? - The Straight Dope
Interesting theory, but the explanation that Cecil pointed out (derived from buck skins – a medium of exchange in the backwoods) seems more plausible. It’s very common for a word to transfer from an old idea to a new one. For example, we still dial a phone number.
Is there any evidence that the use of buck slip goes back earlier than the usages cited from the 18th and 19th centuries?
And what do you propose the pathway would have been to get from buck slip to buck as money? It’s direct in the shortening of buckskin but I’m not seeing how a routing slip would come to mean money.