Bought used early 2011 MacBook Pro, experiencing screen issue

Hey, guys!

I'm new to the Apple laptop world; last weekend, I ordered a used MacBook Pro on eBay for about $650 in an effort to save some money. It's an early 2011 model with the i5 and a 500GB hard drive.

On the first night I bought it, shortly after I plugged a USB keyboard into the the port, the screen started sporadically "flickering". (By flickering, I mean this: little lines jumping across the screen, but not in a way that makes it unusable.) The second and third times it happened were in the next 12 hours. The fourth occurrence (last night) was also directly after plugging something into one of the USB ports, but it hadn't happened in the intervening days when I was plugging some drives in. It's only happened about 4 times since I received it (last Thursday), and when it happens, it only lasts for a minute or two. Then it goes away and everything's back to normal.

So, have any of you experienced this issues or know anything about it? And does it sound software or hardware-related (I'm guessing hardware)?

Is it worth returning the device? I'm pretty in love with it, but don't want to settle for a faulty machine.