Yes, every object, listings, ratings, etc. should be PoS instead of PoW. I suppose the hosts can define their throttling in the contract they make with the seller, so that isn't my decision.

PoB tx usually do require a change output along with it's burn output, and even if we weren't concerned with that, 200 bytes is 1/5000th of a block meaning only 5000 objects can be broadcasted per 10 minute period (assuming everything in the bitcoin block is a PoB tx). In other words, it doesn't scale.