Requiring a steel cylinder to be hydrostatically tested every 3 years is a new one for me but having a shop be annoyed and give me a hard time for buying a tank from someone else isn't new.
The last tank I bought (two steel tanks actually) was being sold for $94/tank less than the dive shop I worked at. The shop had a new store manager but I asked the owner if she could price match. It wasn't a sale item or anything. The other shop was selling the tank for $94 less, always. They sold you the tank, O2 clean, visually inspected and full. The owner told me she could price match. When I arranged to pick up the tank the store manager told me the guy who prepped them was on vacation and I had to wait 2 weeks. Fine. I went to pick them up and there was no visual inspection sticker on them. I was told I'd have to pay extra for a visual inspection. I'd have to pay even more to have them O2 cleaned (they came from the factory O2 clean). To add insult to injury, if they inspected the tank they'd have to drain them. Once drained they'd have to charge me for an air fill. The owner was on vacation.
I told them I'd take them as is. I brought them to the shop who was originally selling them for $94 less. I told him the situation. He asked to see my receipt. Saw they were purchased and picked up that day. He drained them, inspected them, filled them and slapped a sticker on them. I quit working at the other shop and started working for the new shop. The first shop went out of business within a year.
My general attitude towards this is just walk away. If the shop isn't treating you fairly then just walk away. Vote with your wallet. W.C. Fields once said, "There is a sucker born every minute" but the dive community is pretty small. Anyone who is serious about diving will find this or talk to others and find out what this company is like. Maybe the owner was having a bad day and he took out his frustration with online stores on you. I suspect this is basic attitude. It should end up biting him in the *ss in the long run.
Cut your losses and never go back to the shop. Your time is too valuable to waste on this shop.