Short answer: It was a bad idea for the first ten websites that have already done something like that.
The question for you to answer is: What do YOU bring to the table that these roasters would need or want?
Professionally designed, properly optimized website? Doubtful.
An understanding of current market dynamics within the overall coffee industry as well as each localized market? Probably not.
A lot of dollars to sink into a full-blown online marketing campaign to drive awareness and business to targeted consumers for the website? Nah, doubt you can do that either.
Longer answer: (and I'm just hitting the basics)
There is no need for a middle man, better for customers to buy from the coffee shop. The inventory management would be a nightmare. Not for you, for the roasters.
Either you would be a glorified order taker, which (see above) or you would be selling less than fresh coffee to your customers which is bad for the customers and completely undermines every roaster working with you. Also, selling whole bean and K-cups on the same site are two completely different markets. The only way it can work is you have to pick one or the other, have contracts with every coffee shop with a guaranteed order every 2 weeks (to assure freshness) and you would need to pre-pay...
Bad idea.