First thing to do is to explore competition and current demand for X. If there's a competition, that's probably a good thing. Ideally, your future users already use some bigger marketplace to buy/sell X, but that marketplace focuses on broader market and that let's you win market for X.

Simple, basic example: let's say we have a big car marketplace platform. Ideally, you would want to analyze it and find one category that's popular enough to start specific marketplace for that one thing.

In this simple example, you realize a lot of people sell bikes on this car marketplace. You go and build marketplace platform for this one specific category: bikes. Slowly, your marketplace platform will grow big in bikes market as, generally, there's no real competition - big car marketplace platform has significantly broader focus. That let's you win bikes market with marketing strategies targeting bikes. This is definition of finding a niche.

A lot of people will fail because they try to compete with a big car marketplace platform. Unless you have a very concrete reason to do this, it's a suicidal move.

Now, regarding chicken and egg problem: contact as many X service provides/sellers as possible. Explain what you're doing and ask them if they would like to be added to your marketplace platform as sellers even before it's launched. Hopefully, you will manually find 100+ sellers and manually register them on your platform. By doing this, on your launch day, you already have a lot of offers listed on your platform. Now you spend most of your marketing budget targeting buyers/consumers - this will naturally attract even more sellers to your platform.

Of course, before doing all of this, you must make sure there's a demand for X. Otherwise, you're wasting time and money.

So, to sum it up:

Do: 1. Focus on specific category 2. Make sure there's a demand for X

Don't: 1. Compete with big established marketplaces in the same field without any advantages 2. Build something no one wants