Try:
"I've now been cleaning your windows for X years.
In all honesty, I've always considered you to be a
commodity, not a customer. So, in the same way
that Id flog a clapped out stereo on Ebay, I put
you up for sale. Someone bid for you (not much,
but enough that I dont want you any more) so
from next time, itll be someone I dont know
cleaning your windows. Hopefully, that is, if he
can get his R Reg Mondeo running. Oh, and make
sure you keep up payments as hes just out of
Dartmoor after a long stretch for GBH."
NEVER tell a customer youve sold them. Tell them someones helping for a while. Theyll have forgotten you after a few months.