The first is launching with too few products. A shopper who arrives and finds two vendors and a handful of items leaves and does not come back. Before you promote the marketplace, make sure each category you show has enough products to browse, and hide categories that are still empty. It is better to open with a narrow, full catalog than a wide, sparse one.
The second is vague vendor terms. Vendors want to know what they will be paid, when, and what happens when a customer returns something. Settle those questions in your Dokan settings, then write the answers in plain words on the sign-up page. A vendor who has to ask by email what the commission is will assume the answer is bad.
The third is ignoring shipping. Shoppers judge a marketplace on whether the parcel turns up, and the vendor is the one posting it. Tell vendors what you expect for dispatch times, and make it easy for a shopper to report a problem. Last, do not let the community layer wait. Even a short feed of vendor updates tells shoppers that real people stand behind the stores.