As far as I (non-native speaker) can tell, these two sentences have the same meaning:

> 
> I'm just a humble merchant
> 
> 
> I'm but a humble merchant
> 
> 
> 

However I wonder if there is some subtle difference between those two variants I am missing. From the contexts I found the variants in, the one using *but* seems to be a bit more anachronistic and - when used in a modern setting - seems to have a hint of irony/sarcasm to it? Is there any modern context in which one would use *but* instead of *just* for other reasons (maybe indicate belonging to a special social group)?