It is used primarily to certify that something has no swindle hidden.

But, as with a lot of has-been judged expressions, belonging to formal register, they are often used to show some emphasis or irony.

There, using irony and intensified by the question mark, this expression hold a sceptical meaning.

For your translation, I think you should keep the question structure.

Simply using honest.

Would they have gathered here to build an honest trading port? Don’t make me laugh!

Using superlative.

Would they have gathered here to build the most honest trading port? Don’t make me laugh!

A good translation for this is “As honest as can be“. It essentially means that you couldn’t be any more honest.

“As respectable as they come” is a similar translation that also works.