Thanks Roxy. Everything you say makes sense, but why ask when they haven't made any proper enquiry (dates in particular) ?
Oddly enough, a half hour later I got a "newsletter" from the listing site concerned warning all advertisers about possible scams:
Valued Customers,
We would like to make you aware that all of the rental properties on Rents4vacations.com have been receiving the same rental inquiry, which seems very suspicous. Just a reminder that you can never be too careful when dealing with prospective renters. If a prospective renter is proposing unusual payment arrangements or wants your confidential financial information, you should be very wary. For further tips on how to avoid fraud, check out
www.fraud.org. If you have any other questions, please let us know.
I think I was right to be suspicious, but on reflection, in this particular case, I suspect that they were simply harvesting email addresses.