On what basis is it a moral harm?

I don't think your specific example illustrates moral harm. But there are definitely advertisers that do. We could for example say how BP spends a few million on ads to buy public good will and talking about how they are cleaning up the environment while they increase their pollution levels by expansion is morally reprehensible. Such advertising is quite common.

A transaction is good if both parties benefit, but in this transaction I end up with a product worth less to me than I paid.

Any extra persuasion on top of listing features and asking price is an attempt to shift the potential customers away from rational cost/benefit analysis, leading them towards making a suboptimal choice. In simpler words: it's making their lives worse.