Yeah sounds odd, if it seems odd in anyway walk away dont bother as it will prob end up going south!!
Last lot of work we sold we had a list of 30 odd names addresses phone numbers on a spreadsheet and last date cleaned and access notes etc etc
Drove him round in our van and showed the guy where the houses were and ticked them off sheet, then we got him to select a customer off the list who we phoned, did the usual politeness chit chat then explained about about change over, kept going till guy was happy.
If you/they have nothing to hide they should be willing to help in a smooth transaction rather than avoid questions or change track.
On the other hand we have spoke to other windies about buying work weve had no end of trouble getting simple details, like names, contact numbers, full address, last clean etc etc Its amazing that such simple details are not asked for or kept by some window cleaners!!