he is interested in selling. Two days ago he asked us if we wanted to buy it. I'm in the process now of researching what I think the value is. He has not told us a price yet, he is supposed to get back to us. I figured i would do my own research and come up with my own price.
I would continue to do your research, but get him to give you a price rather than making him an offer.
You might try finding any recent sales of similar properties. You may have to piece it together, say from a convenience store/gas station sale and a land sale. Loopnet does have the ability to get recent sales, for a charge. You could also try contacting some commercial agents in your area. As you're driving around, look for signs on buildings or stores, and you should be able to locate some of them.
That's a great tip from Jon. No matter what type of real estate, always try to get the price from the seller. You can always negotiate down but it's a good possibility they may ask for less than you were prepared to offer.
I agree with the previous two posts. It will save you a lot of time by having the owner give you a price where you can start your negotiations. Still do your due diligence to research the market value for that type of property. Look at properties that are both occupied by a tenatn and vacant. As a user of the property, it would make more sense to for the purchase price to be based off of the property if vacant. If you used the income approach, you are basically valueing the income stream it creates. However, use whichever means of valuation that is in your benefit.