Keeping a wide range of products in your store is an expensive affair. A way out of it is to sell for companies on a consignment basis, but not all companies follow this method. Most big companies like Korg will ask you to but their product at whole sale rates upfront.
To start with a repair shop is a good idea provided you know your stuff. And providing repair services with a series of brands is even better. You must understand that the margin from the selling price is the only margin you get, so an add on to it is always welcomed by store owners.
Another good add on would be reselling old and vintage stuff for people around. There is a big section of enthusiasts and professional who are hungry for the vintage stuff which is hard to find. Tapping on some vintage pieces will add flair to your store and also add on to the inventory. Making heavy profit from a guitar store is a bit difficult, but not when your catalog is superb and you have several revenue generation sources under one roof.
Read the whole article here - http://ask.metafilter.com/67282/How-much-does-it-cost-to-open-a-guitar-store
