I dont go along with the contracts scenario, just my opinion.
You are buying the goodwill, and also if you go collecting with current window cleaner, you will see them being paid and the seller can just say to the customer they are thinking of selling and could they confirm the frequency and also the cost, trust me thats worth more than anything.
There is alot of rubbish work out there for sale, but that is upto the individual to use common sense before parting with any money.
I have over £200k of work which is regular work, as i dont do one offs, and none of it is on contract.
If i was selling i would show the work being cleaned, show my books or computor accounts, introduce to customer, that would be good enough for me if i was the one buying the work.
The only problem i ever had buying was a round i purchased in sale, manchester about 20 years ago, but that was because i was too trusting and hadnt got the time to walk round the work with the seller.
I learnt my mistake from that day, and so did he when we next met