It's sad that these scumbags invade all the internet now. Best to - as you have - keep your head on your shoulders and don't pay any money upfront.
Gumtree can be great - I found new homes for the five kittens my grandad's cat had (after taking in a pregnant stray cat - that we've now had spayed!) within a day of putting an advert up on Gumtree. We didn't ask for any money, just good homes. But, looking at Gumtree, it seems that many people want money. It's sad - they should just advertise in their paper and leave sites like this for more charitable endeavours.