I think selling computers is definitely worth it. How often do you have a repair that costs more than what your customer is willing to spend? If you have a piece of cost effective hardware just sitting there, begging to be purchased, you can walk the customer to your display, and explain their options. Warranty the unit for 60 or 90 days, whatever you feel comfortable with. Offer an additional warranty period that is valued at whatever your cost on the machine is. This way if the machine bites the dust, you've made your profit, and the extended warranty paid for the replacement.
I don't have my own shop, i work out of a home office, but a large chunk of my business is wholesale refurb computers to computer shops. I have a few clients with extremely small shops that purchase maybe 40 units a month and make a killing. It's definitely worth it, if you get a good price and execute properly.