I would contact the seller before selling any of
your cels. Ask him if he will part with the cel for
xx$ and that you don't have the money yet - all what
you've written here.
The cel isnt officially for sale so there's no need
to hurry for you and the seller, all he has to do
is wait until you have the money if he wants to sell
the cel to you.
Its always the best thing to do to talk plain text
with the people.