In my experience, a marketplace always requires large investments in marketing (((the problem of such a business is a chicken-and-egg issue. Clients will not come to the site until there is an insufficient selection of goods and sellers, sellers will not come to the site until there are not enough potential buyers. It’s more convenient to start by attracting unique sellers and products, but for this I would use direct sales, offer special conditions for the first sellers, and then launch advertising on products and product categories in order to gradually gain a base of potential buyers.

Not enough information, do you provide services or sell products?
If you sell goods - the best way is to add them to all kinds of marketplaces, such sites have huge traffic and you can find new customers there. The main thing here is to stand out among competitors - make bright and attractive product cards, people buy the picture.
If you sell services - here is a little more difficult. Start by registering on review sites like Capterra and Trustpilot - ask your customers to leave reviews and complete your company profile. There are also sites-directories, they have very high traffic, from Europages and Yellowpages to more specialized, such as ReStaffy for accounting and law firms.
This way you will get both reliable links to your site and additional traffic from potential clients.
I also advise you to find relevant bloggers and collaborate with them - a strong recommendation always works better than any advertising, especially if the blogger is trusted by his audience.

The discount will certainly attract people, but are you willing to pay that much for a subscriber? Try putting a qr code to go to your social networks on the price tag. I would add there the phrase: “Be the first to know about promotions!”
We’ve also used a random lottery - as a way to get people on socials. Along with the qr-code place information about the lottery among subscribers who left a comment under the last post - 3 gifts from your assortment, drawn among a couple hundred people for us is a much more economical solution than to give everyone a 10% discount, plus it greatly increases the activity under the post and viewing stories, because people are waiting for the announcement of the winners.
Good luck to you!

It’s very unpleasant, but you have no reason to worry. From a technical point of view - the buyer has received the parcel (so whether it will turn out so with the courier service), there is no precedent for a refund, because there is no return of goods.
What I would worry about here is reputational risk, the buyer can actively spread the word about this on socials.
My good advice: keep in constant contact with the customer until the circumstances are clarified, not on his call - be proactive and report the status of the investigation with a courier 2-3 times a week.
If the customer turns out to be right - you can always make it up to them with a discount or some nice bonus. The main thing is not to put out the fire with silence.