Gumtree buyers are just normal people, same as ebay buyers, forum buyers or newspaper buyers.  There will always be people that will lowball you, ask stupid questions or be tyre kickers - but that is part of buying and selling.  I know some people get hung up about lowball offers or questions and the like from selling on various sites, however i just politely refuse the offer or answer their question - interestingly i have had a few people buy stuff (not necessarily pinball related, although one was) who based on first communication appeared to be time wasters, but then came through with the cash at advertised price.

Obviously do not sell to someone who is on an oil rig, who has an agent who will ship and will transfer money via western union. As long as the cash is in your hand or account before you hand over, there should be no issue.  If you want to avoid a few tyre kickers, you could speicify very clearly in the advert that SMS and emails will be ignored, and ask that they phone you.