For the supply in a two-sided marketplace, is it initially more valuable to have a couple of high-profile people, or a lot of standard-level people?

Good question.
There is no absolute correct answer, as both options have their pros and cons. At the end of the day, you need to bring in as many users, on both sides (experts and people using their services) in order to create traction and gain revenues.
So, if the "Celebrities" are willing to endorse and advertise your platform, then it is better focusing on them, but in most cases, they won't do so, therefore, it would probably be better, and easier, focusing on getting regular (but quality) experts. Also, take into account that "celebrities" usually have less availability and are more expensive, which will affect the level of service that you offer your users.
Good luck
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I think this is not a good idea to have a couple of high-profile people for your 2 sided marketplaces advertisement. Because celebrities generally have less availability and are greater high-priced, with a purpose to have an effect on the level of service that you provide your customers. I think you have to think about the marketing strategy. You should understand how to attract both buyers and sellers.

The good idea is, you have to choose the right professionals, it will save you a lot of time and money. Expert SEO and SMM professionals will promote your website effectively. Another way of growing marketplace popularity is a high-quality website, that is, the fast work and interesting design and, of course, it is the absence of gross bugs including.
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When building a platform out a platform, the most important aspect initially is to be able to create core interactions among the initial users. These core interactions make the platform valuable, and vastly increases your chances of developing viral growth loops, as opposed to word-of-mouth effects.

To help you distinguish the difference between viral growth and word of mouth, here is a thinking.
Word of mouth will happen when somebody hears that Mark Cuban is on Clarity and she will tell her friends to check Clarity out; it is newsworthy. You will get sign ups out of curiosity, but those new sign ups will still need to interact within the platform to create value.
Viral growth will happen when users get more benefits by having their own social connections on the platform, and therefore they will invite their friends to join. For example, if Mark Cuban was on Clarity and Clarity was offering a "group chat", a single Clarity user might decide to invite her entire startup to join Clarity, so that each can connect to the group chat with their own accounts and ask questions to Mark. This positive network effects differentiate viral growth from other traditional marketing effects, and viral growth relies heavily on the existence of core interactions in the platform.

Finally, here are tiers of value your producers would create early in a marketplace
Most valuable scenario: You have high-profile producers and they interact. I.e. Mark Cuban is on Clarity and he takes calls and answers questions.
More valuable scenario: You have producers (high-profile or low-profile, doesn't matter as long as they can produce value) and they interact.
Less valuable scenario: You have high-profile producers but they don't interact. In this case, more producers would continue to sign up to become associated with Mark Cuban, hence you get some viral growth. You would rely on your low-profile producers interacting.
Least valuable scenario: You have low-profile producers but they don't interact. This would signal the future demise of the platform unless you step-change the core interaction volume in the marketplace.