While I agree on these tips unfortunately a big one is not very PC so won't be mentioned,basically if you hear the persons voice is of west african original you need to be extra careful,In most cases it's a scammer.
Now don't get me wrong I know there are many good Nigerian people out there but there are also unfortunately too many who just make a business out of scamming people on classified sites in SA,OLX is also flooded with them.
They deposit stolen cheques into your account instead of doing a proper bank transfer and send a courier to collect your stuff for free,they also advertise as being in remote areas like Northern cape,Upington and other remote areas when they are actually sitting in Hillbrow or elsewhere in JHB.We've even busted a syndicate operating out of a huge mansion in upmarket Houghton.
Yes there are many local scammers too including one well spoken well dressed local guy who will swop out your expensive goods(cellphones,jewelry etc) with a cheap dummy during the sale then says he needs to go to an ATM and never returns because he already has your stuff,but there are also thousands of honest local people so the ratio for foreign people is just too high to make it worth even trying with them.