My guess is the person wants to trick you to get away with just paying little money, to exploit you. That's why you alway got those excuses.
Eventually, it might be better to call (or write an e-mail if you prefer that over calling just like me) and say "Listen, you still did not pay the remaining $30. You gave me $20/$50. I am waiting very long for this now and would like to finally get the money. If I won't get anything, I will come over, give back those $20 and take the computer with me. Theoretically, it still belongs to me as you did not pay the full price."
I would do it that way, it takes some courage but I think this is the only possible way of finally getting what you deserve.
_________________
Diagnosed with Aspergers.
BSP-errors are awesome.