Ask HN: Costs of verified purchase review tags to the seller

My PhD student is working on a project involving verified purchase reviews. The major benefit for sellers is that these tags increase credibility of the reviews. We were debating about the sellers' costs of implementing such tagging. Clearly it requires linking every review to the purchase, which can be costly to implement. Competitors can also use the number of verified reviews to arrive at an estimate of the units sold. But these are just my uninformed thoughts and I could not find any concrete data or informed opinions. Clearly there are some barriers to its adaption as not every online retailer is doing it. I would love to hear what you think/know about this. I value your inputs a lot.