Back when we had a sales tax license, we opened accounts at the places where plumbers & electricians buy their stuff. The wholesale suppliers did not care that we were in the trucking business.
My brother and I did a quoted at $12,000* oil/hot air furnace job for around $4,000. Local law only required the oil supply lines and electric be hooked up by licensed pros. The work we did was far easier technically than doing a Hinckley valve adjustment.
Sure is nice to get everything wholesale.
If you're set up for sales tax, then...
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*The markup on furnace jobs around here is sky high. The only thing I can figure is the installer must own it for life with regard to liability issues and insurance costs? Dunno.