Ebay (like gumtree) depends on the seller - if they are highly rated, then likely the buying experience will be positive.
Shipping/clearing customs is where things get a bit...ermmm....grey. @NorioDS recommends Aramex Global Shopper (check this thread) - basically use a shipping company that sidesteps SAPO by handling the customs clearing for you.
New items are subject to VAT (15%) and you might have a 1% customs handling charge added. But there's no other SA specific import duties on guitars.
So landed cost = price + shipping + vat + handling charge.