well i think a private seller will more likely know the value of the item and how much it can go for. If it is in an LFS they probably won't have an exact idea of what it's worth. Don't get me wrong there are some LFS that know their stuff, but that number is very small. So IMO it would be easier to talk them into possibly trading it for something or trading it for something of lesser value, or possibly telling them that their price is to high and working down. So an LFS would be more likely to not get the full value of the fish during the transaction, and a private seller would most likely get full value for it give or take a little bit.
Text or PM works for me Frank