I'm going to chime in on this as well. Some time ago, I thought about purchasing an existing business - or possibly opening my own. Some considerations that I ran through were:
Location - where was the retail operation, where was it postioned? Having it on a Main Street instead of a side street was a plus. Had to consider customer parking - was there public parking or own lot.
Store - I was told it was better to OWN your property rather than rent/lease. Landlord could pull the rug out from under you and force you out, sell the property or increase rent year to year.
Diversification - Better to have several 'interests' or product lines rather than one, due to each hobby having its own 'season of interest' Think R/C- both surface and air models, slot cars, model kits (especially kits like Warhammer, Military etc.), Die-cast, ancillary supplies. I would also have to carry multiple manufacturers of one product line - i.e. Atlas, Lionel, MTH etc to effectively meet the needs of my customers.
Internet presence - Needs to be strong, simple with up-to-date stock information.
Suppliers - As I found out, unless you sold over a certain amount, I would never be able to buy direct from some manufacturers - I would have to go through distributors, negotiate invoice pricing which would determine consumer pricing in order to make a profit.
Inventory - On-hand stock vs. 'Pre-order' or Ship-to-store orders. And then gauging interest in certain products to keep stock on-hand and available for walk-in traffic or internet orders.
Marketing - In general, it's great to market to your hobby customers specifically, but you also have to create a presence and market to a general audience to drive that foot traffic and internet traffic reaching beyond your circle of customers.
