By the very loose definition that was recently ratified, anyone who plays games is a potential addict. But more seriously:
1) If you avoid all social contact in favor of playing games
2) Ignore or let lapse responsibilities to play games
3) Ignore personal hygiene/health to play games
4) Put yourself at financial risk
I would say all of those happening at once is pretty much a case for some examination of one's priorities.
Then you get into gray areas. Some people play games for a living, either professionally in competitions or with streaming. They are likely addicted to gaming's endorphin/adrenalin highs, but since they are 'supposed' to be doing it, they wouldn't qualify.
There certainly is. Game designers specifically build features that implement "feedback loop" and "game mechanics" to keep you playing. Say, a game provides a black box that accept some kind of input (hit a balloon, pull trigger while aiming, etc.), applies its internal rules, then provide some kind of feedback (somewhere between negative and positive) with all kinds of effects (visual, auditory, motion, etc.)
Gamers try to figure out the internal rules, and try to maximize his output (more positive and less negative feedbacks). Gamers gradually figure out the right inputs, receive more positive feedback, which reinforces their efforts to do even better. This is probably a form of addiction to some degree. If you stop eating, sleeping, going to work, in order to stay in this feedback loop, then that's probably a major addition.
I used to play poker and once came to the conclusion that I simply couldn`t stop. Now I play but try to control it. The best way to prevent an addiction to video games from forming is to force yourself to take breaks, and to not let gaming interfere with one's social life, or even personal life. Some people have a harder time controlling this. If you feel that your gaming habits are taking control of your life, take a week or two off from gaming. Finding other hobbies, or keeping busy will also help you.
Ok, it's hard to answer this question only by knowing that you sell stuff you don't use to play games etc. It doesn't look like a big sign of game addiction for me, but if it gets worse, for example if you can't control your budget or you can't manage your time because of games, then that means this is a warning sign. It seems ok just for now.
You have an old clunker of a car in the driveway, that you no longer drive. Selling it in order to help purchase a new car does not make you a race car driver. It only means you are not hoarding useless stuff.
You are only an addict if you must sell stuff to buy pc stuff that's not part of a wanted/needed upgrade path.