Hmm you say boiler but since you said 30gal makes me think you mean water heater.
I think that 10-15years is the MAX recommended life for most water heaters. Some last much longer, and some much less.
I have never owned and oil fired unit but I do know they are pricier than gas or electric units. I would be hesitant to buy a used one at 6years old. I would prob only consider it if the tank was stainless or something other than steel. Who knows how much life is left in a used tank.