Would be nice to just type just "/sellall" and not have to remember / check the spelling for my highest shop. EX: "/sellall srfarmer" is annoying I don't know of any reason to sell specifically to a lower rank shop.
The only reason I ever sell to a lower rank shop is to check prices for other people, but I think /prices covers most of that so I would agree with you that the extra argument is redundant.
It would be nice if /sellall just went straight to your highest shop. We could keep /sell shops for people that want to check prices without /prices.
I think there's recently been an update where you can type /sellall <rank> to sell to a particular rank, or just /sellall to sell to your current shop.