First of all, this will come down to branding and how you wish to position yourself relative to the vertical (niche) you wish to operate in. Things such as market research do help to a certain extent but it will also come down to what that luxury item is.

For instance; in the sense of luxury watches or jewellery, the brand will play a greater role owing to the provenance, and the target market who wish to pay/acquire those particular items will put more weight into the brands carried as a way to validate the brand of the place that stocks those particular items.

This leads to the secondary consideration is the particular clientele you wish to approach. You'll notice that in the areas regarding luxury you might find two major categories; sellers who wish to offer the absolute lowest price for luxury items thereby making them more accessible, or those that offer more of the concierge service and charge accordingly. In both cases, you will be able to tailor your approach from your presentation, stock levels/selection, pricing, features (things that you want to stand out on against your competitors), and overall service delivery which makes your marketing and sales channels much easier to select and focus on.

Once you have a clear idea of those basic questions, the next stages become much simpler as you aren't trying to answer HOW to make money, you're answering the question of WHO are you going to make money off.

Ultimately; your clientele and business model can be profitable if managed properly and if you keep that in mind; a marketing strategy can be made that is actionable and sustainable.

If you wish, happy to talk more and provide more detail should you have more follow-up questions.