I think the strongest version of this idea is worth taking seriously, but I would narrow the claim.
I would be cautious about saying the cave markings were literally marketplaces, contracts, or ledgers in the modern sense. That may be too specific for the evidence. But the broader idea is important: some of these markings may have preserved survival-relevant information, not merely “art” or “proto-language.”
Animal signs, dots, lines, handprints, and repeated symbols could plausibly have helped store or transmit information about herds, seasons, routes, group presence, access, obligation, or shared memory. In that sense, they may belong near the origin of economics: not money, not prices, and not formal markets, but durable records of productive knowledge.
That is where I think the idea becomes interesting. Before humans had written language or accounting, they still had to manage food, risk, memory, cooperation, and future need. If a cave marking helped preserve information that increased a group’s ability to survive or coordinate, then it was already economic in a deep sense.
So I would not jump straight to “prehistoric Craigslist.” But I do think it is worth asking whether some cave markings were early records of productive capacity, resource knowledge, or intergroup coordination. That could be a serious research question if framed carefully.