Why are they asking for topup? Just because its more new? Generally Bs are a margin more expensive than kellies; used Bs in pretty beat up condition - showing corner wear, handle darkening, scratched hardware etc etc are normally priced around 7000 usd on average. Kellies in similar condition can go down to 5000. Also, with the current market climate, a k35 will be in less demand than a b30 so the markup makes even less sense. Of course I dont know the specifics on condition...
Also swift leather looses its structurе ery easily. Especially in size 35 kelly.
B30s are in demand now. Unless this swift kelly is something you actually really want (as opposed to just settling for the next best option) I would suggest selling your b30 and then getting something you really love.