If he hasn't been there for the best part of two months, do you not think they're going to tell him to get lost when he tells them he's packing in, but is recommending you to carry on?
Depends how he words it, I suppose.
Dodgy to buy - the work or the area?
Quite frankly, if he can't service it and couldn't see in advance that he'd have to get a replacement ( are you the first person he's tried to sell it to? ) he deserves to lose it.
I never said this, but leave it a few weeks and then canvass the place yourself. After the first few, word will spread and they'll come to you.
BUT, they've probably been phoning round trying to get other window cleaners, and if no-one's taken on the work, why should you?
Sorry mate, my thoughts are a bit garbled, but you said urgent!