Questions, questions
If you can, find out why the vendors are selling. A new job, a divorce or new baby could mean they want a quick sale. Vague answers could mean problems with neighbours or lack of serious intent.

Have they found somewhere to move to? You could avoid a tricky chain.

What work have they done to the house? Ask to see any certificates or planning permission documents.

A prepared seller should be able to show you past utility bills to give you an idea of running costs.

What fixtures and fittings will be left? This will be decided in detail later but it's worth finding out roughly what's included in the asking price. See Making an Offer and How to Negotiate for more on the nitty-gritty of settling on the purchase price.