Starting problem

I just bought the vehicle listed above used. When I try to start it, it will click one time but it will not start. It started right up when I first bought it. It is not the battery. I assumed it was the starter so I took it to auto zone and O'Reilly's to have it tested. Both places said the starter was good. I put it back on the vehicle and tried it again. One click and nothing. I cleaned the battery post just in case. No change. I went under the vehicle and tapped on the starter. It started. Now it will start sometimes if I keep trying a few times. It will click once then start after about four or five tries. Is it possible the starter could be faulty even if the bench test was good? And if were the starter relay, would it click at all? What is going on? Also, it runs rough and sluggish. No power on acceleration. Would that be tune up (spark plugs)? There is no check engine light on right now. It was when I first bought it. It did not come back on after I disconnected the battery.