I like that the etiquette for posting replies in the marketplace category is being discussed in terms of what would be appropriate in a physical store. There are times when it makes sense to offer advice about alternate solutions, but if a discussion goes too far off in that direction, it should be moved to the support category.
This is getting off-topic, but I’ve been looking though the closed marketplace topics. One thing that stands out to me is that it is hard to know if the work got done, who did the work, and whether or not the customer is satisfied with it. Here is an exception to that: [PAID] Custom Header to Match WordPress Site. Maybe topic creators in the marketplace could be encouraged to create a follow-up post before the topic is closed. When possible, a link to view the completed work would be nice. It would make for more interesting reading, and also give us a sense of how successful the marketplace is.