There is no law or regulation that suggests you cannot sell monetary instruments to non clients. There are record retention requirements that kick in if you sell them for cash between $3,000 and $10,000 inclusive regardless of who the purchaser is.
Nemsi has focused the issue on restrictions your bank might impose voluntarily; e.g. a self imposed prohibition on sales for cash to non depositors or non customers in general. That is common.
In effect, if there is a restriction it is self imposed and whether the "exchange" of an on-us customer check for a cashiers check violates that restriction depends entirely on the language your bank chose when it penned the restriction.
_________________________
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.