There are 2 things to look at when doing a purchase: Assets and Goodwill.
The Assets are exactly what they sound like. They are the inventory, the building (if it is owned), the equipment and furniture, etc. All of that can be valued.
Goodwill is what the current owners or the purchasers are willing to pay for the intangibles: customer base, the name, the location, the reputation, etc. If you say that the shop has been run into the ground, then there will be very little goodwill. In this case, you will probably run into a problem as the current owner will probably think there is more goodwill than the potential buyer thinks the shop currently has.
So, when valuing a business, those are the 2 things you have to look at. One is very cut and dry and one in far less tangible. Typically, either the owner or the proposed buyer will come up with the figured that they think are fair and will throw them on the table. Negotiations will go from there.
Make sure to split the 2 up, though. Dont lump the two together. Keep them separate because goodwill is something that you will have to determine how to absorb that into the business once the sale is complete. You pay for it but dont have anything to show for it other than customers walking in your door.