First of all is it a transferable Sten ?   Origional tube Sten's will bring 
$3000.00 -$4000.00  ,,, Sten's that have been built using a aftermarket
tube will bring $2000.00-$3000.00,,, 
But be sure its a legal transferable gun,,, if it is not.. Id stay far away from it...  And yes a $200.00 tax must be paid when you submit your forms to the ATF.. then be prepaired to wait up to 6 months till you get 
your stamp back... then you may take possision of the gun....
you better not get caught with the weapon with out the stamp..(paperwork)....