This is likely a local rule put in place by your LO. In fact, by the "letter of the law" (so to speak), you can mark your 8 ball pocket with anything including, but not limited to, coins, paper money, wallets, purses, a botthe of mustard, a shoe, chalk, or pretty much anything else as allowed by the team manual. Local rules may be put into place that would over ride this rule, for example, apparently in at least a portion of California it is not permitted to mark your 8 ball pocket with any type of currency. On a national level, however, this rule is not applicable. I've seen these local rules come up to bite some teams in national tournaments. One thing I'll do when playing an opponent that has been particularly obnoxious, if the opportunity presents itself, if my opponent shoots and misses the 8 and leaves his marker on the table, if I shoot the 8 at the same pocket, I won't mark the pocket and just let his marker sit as the marker. I love it when they yell I didn't mark the pocket. I simply show them the rule about the pocket already being marked.
To the OP, I'm pretty sure in your situation that clown was try to take advantage of you being unsure of the rule. I hope you get to play him again.