Will customers refuse a liquid tank if it is stained on the inside?

Happens all the time, one tank repair facility lost two family members when one jumped in after the other...You need confined space training and follow the system, obtain an entry permit and have a lookout with a rescue device standing by while you are inside.

The discoloration appears to be on the topside of the steam panel and could be the result of improper product heating at some time in the past that discolored the stainless.

Passivation might remove it. That's a fog of Nitric acid and hydrofluoric acid that is misted into the trailer until the stainless appears to rust. then the trailer is washed. This cleans and seals the pores of the metal. A few companies I worked for passivated every new trailer before putting them to work. The rest of the tank appears to be in good shape so this might be from overheating the steam panel and has been there for along time.

Some repair facilities have the machine that would polish the stain away, similar to the way the ground off welds appear in your picture but this is labor intensive [costly] and if they are not getting rejected they may just leave it alone.