The suggestion there was that someone might suggest/narrate the market stand, as providing an opportunity for a Diplomacy check ("Good melon vendor, may I use your market stall as a ladder to get over yon wall?").I thought that was some sales stand at the wall. I admit that my example was based on it, but it was not meant to refer back to it.
When to "say 'yes'" to this sort of framing context for a check, and when to call for a check as a precursor to the framing (so eg requiring first a Streetwise check to know where the market stall is near the wall, and then the Diplomacy check to persuade the vendor to allow its use as a climbing frame), is an issue of GM judgement around situation, stakes and pacing in a skill challenge. There's no algorithmic answer (in my view).