When people hear the word "marketplace" as a consumer, the first thing that often comes to mind is an app with a map; Press the button, and something comes to you. However, it's wrong to assume that all marketplaces should necessarily be mobile.
Marketplaces is a very broad term and there are many different ways to cut it. One of those ways is to consider whether the marketplace is on-demand or planned. Depending on the type of service, this consideration greatly affects the design of the marketplace. For me, it comes down to who the user is and what their flow is as they are requesting the service or product offered in the marketplace.
For example, marketplaces like Uber, Caviar and Bloomthat are great on-demand examples. They share the characteristics of frequency and immediacy. In these cases, a mobile-first strategy (or mobile-only) is very applicable and almost necessary because the request has to come from a device that's always available to you. Additionally, the information that a user needs to provide to request the service tends to be minimal and can easily be found on a mobile device. And lastly, the user is not choosing a specific person to provide the service.
In contrast, there are companies like Laurel & Wolf, Honor, and Care.com which are more planned marketplaces. Here, these are things that people may want soon but not usually immediately. At the times that they're looking on the marketplace with intent, they've usually set aside some amount of time so they can have access to a laptop or a device with a larger screen. Depending on the type of service, there's also a lot of information that needs to be provided, e.g. specific room dimensions, pictures, links, or detailed notes which takes time to put together and to input. Lastly, the users here need time to think about the specific people who are providing the service and whether or not they trust them with their homes or parents or children, which is a serious decision.
There's definitely an argument for having a mobile presence to support some part of the marketplace transaction. For example, every marketplace creates communication between the two parties and mobile is the easiest way to facilitate that. An app that allows for quick questions or photos to be exchanged on the go might be very useful. However, in thinking about a roadmap for your marketplace, the on-demand v. planned framework is a useful construct to develop a mobile product strategy.