You have two costs of acquisition.
Take an $85 order at a 13% take rate: you keep about $11, so that is roughly what a buyer's first order is worth to you. An active seller keeps producing orders for years and can be worth many times that. One ad account, two very different values per click, and the platform reports both the same way.
Liquidity is local; budgets are not.
Below a threshold of live listings in a city, buying demand is waste however cheap the click, because the request can't be filled. The same campaign can be right in one city and wrong in the next, and averaged into one account figure both disappear. Geography breakdowns are part of what Adgent reads.
Trust can move the transaction off the platform.
A randomised trial on an online freelance marketplace found that once trust between the two sides was high enough, more of them took the work off the platform, offsetting the revenue gained from better matches. A match your ads paid for is not always a transaction you bill.
Source: Gu & Zhu, “Trust and Disintermediation”, Management Science, 2021.
Bidding against your own supply.
When eBay switched off brand-keyword search ads in a controlled test, the traffic came back through its own organic results; the paid clicks had bought little it wasn't already getting. Marketplaces routinely buy branded and category terms their own listings already win.
Source: Blake, Nosko & Tadelis, Econometrica, 2015.