Derekkawaii :
Oh one more thing, is ok to buy used gpu that was used in mining for a bout 3 to 4 months?
Most likely, however with any used electronics there is a risk, especially if it's been used 24/7 for mining.
Most miners will actually underclock the GPU a little bit to get a large reduction in electricity usage (and heat generated) for the loss of only a small bit of performance.....this is good for the health of the GPU, but the rest of the components still have the hours on them. So, make sure it's all in good working order before accepting it, and I hope you're getting it cheap.