Then the person at the shop looking into tanks is an idiot.
That staining in no way affects the breathability of the gas. If there was chunks if rust, pitting, or darker heavier rust with texture beyond staining that is different.
Shops get spoiled looking into brand new tanks thinking all steel tanks need to look like that to be good, but slight discoloration on otherwise good steel is perfectly acceptable. Then there the possiblity that they are just trying to generate income by claiming the tank needs to be cleaned.
Their compressor their rules, I guess. I’ll been in that exact same spot with shops, but I do my own cleaning way beyond what they are capable of doing. I bought a brand new tank (E7 3442 100) from a shop once and they gave me two free air cards with it. I only used that shop to fill the tank. After a year I took it back to that shop and we pulled the valve off to find it caked with brown rust! WTH!!
They accused me if going somewhere else and/or using the tank on a charter boat, etc. I told them they are the only ones to ever fill that tank and they basically destroyed the inside if my tank and I was pissed. Their compressor was terrible. They wanted to charge me to have it cleaned plus the VIP plus the air fill. I told them where they could stick it!!!
That’s when I designed and made my own tank cleaning tools and it came out spotless. The only time I set foot in that shop again was to gloat and show little miss sales lady my cleaning job. They were actually impressed, and when they wanted to put a VIP sticker on it and fill it for $25 I told them I’d take the sticker for free for my trouble after they f_cked my tank and they could keep their air fill. They took it as an insult of course and after a few choice words back and forth I left and never set foot into that sh_t hole again, F ‘em!!