Any tips on how to structure a first-time wholesale deal?

I've identified a local mom & pop retailer that also offers wholesale prices on a particular premium product (which they import from overseas themselves). I'm interesting in buying in bulk from them, re-branding and retailing online to a niche market that I think they are overlooking (or maybe can't access themselves). Any tips on how to approach them, or pitfalls to avoid? There might be potential for a joint venture if it was structured properly.