Punter

In British slang, this is usually meant to refer to someone who gambles – who places bets with a bookmaker. There are some other usages: someone at a bar or a prostitute’s client.

Why I Looked It Up

From No Bullsh*t Strategy:

Would a punter be able to “see your strategy” in the business just by looking at it?

The author of the book is British. And by “punter” here, I think he means speculative investor.