Depends on your customer base and where you see yourself going.
Re-buildables are trickiest to find in B&M, but that is probably because the market is small and the profit margins probably smaller (although B&M Muji must be lucrative).
It would be great to have a knowledgeable, friendly, comfy place to stop, vape & buy. But from a business point of view you need though put and sales, not a drop in centre.
What about a coffee machine with a few seats etc. visitors could toot & drink (not sure if license needed for this & crisps, confectionery etc.)
Money will be made on high value consumables - coils & juice, realistically for business you may get more joy with entry level stuff?
As 'experienced' vapers we want high end, shiny, re-buildable (some), mechs (some), etc etc. BUT POTV is a small % of a small % of vapers.
Tough balancing act IMO. Buy a broad range in small qty. so you can understand your market and re-stock desirables quickly. Ones that sit will have less impact if qty is low.
Good luck - where are you opening?