I would be careful on this one. Amatuer artists are very quick to tell you they cannot get what they want locally and equally quick to tell you they don't want to pay retail prices.
Take a look at these two websites
http://www.artsupplies.co.uk/
http://www.saa.co.uk/shop/default.aspx
Then decide if you can compete on price with them.
Unless you are buying in very large quantities then you will have to use wholesalers (I can give you details if you PM me). This puts you at a disadvantage on price.
I stock some art materials but it is very limited and it moves very slowly. I have made a point of getting to know some art tutors locally and offer their students a discount. This does bring in some framing jobs and from that point of view it then becomes worthwhile.
What I think you should be looking at is not doing this for your existing customers but think about what new customers you can gain by stocking art materials.
The range of art material is vast and artists are very fickle in what they buy and the brands they like.
On the plus side once you have bought it in and put it on the shelf then it pretty much sells itself as the customers select what they want come to the till and pay. Compare that with spending time measuring artwork, deciding on mount colour, frame choice, shall we have non ref glass? etc. etc. Then you have to order moulding, mount board, time taken to do the job and so on.
You will also need to display the materials and to get the display stands then you will have to buy all the stock that fills them!
A good knowledge of the materials helps as well which I do not have but my wife is an artist so can help me out.