If you are considering selling any items on GUMTREE (and probably on other sites as well) you should take a look at the following post on this blog.
There are a number of scamers out there, and a number of them seem to reside in the Malasya - Singapore area, and they are very cleaver at getting you started on talking to them. Once they do get you to respond, they will step up the pressure and try to get you to send money to them. Of course they will make all sorts of claims about dealing with PayPal and Western Union, but the end result is to separate you from your money.
If they do get you to send money, you can just kiss it good-bye. There is no known way to get it back. It will be gone.
Go have a look at the post and be for-warned.
Here are some links to other articles about the same problem.