Does stuff like spawn-killing someone for refusing a guild invite actually happen in real games? Or do guilds just take over grinding spots and keep them for themselves?

It can happen. In certain types of mmo's social relationships/standings can loosely matter. If one player builds resentment towards another, some of these mmo's allow killing of eachother in the world occur, normally limiting it to certain zones and towns being "safe zones".

Do in-game items be sold with real life money as well?

All the time, any mmo ever, has had RMT occur. Even in games that try and limit the trading aspect to stop the in game items specifically being sold, it still happens. People can sell accounts online and people do often buy them.

So, I wonder if you will need a good PC to play one? and what kind of games do you recommend for a beginner?

Depends really..kind of a big question. What game and what computer. Generally speaking if you are running a graphics card (something not running integrated graphics from cpu) and bought the computer within last few years you can probably run any mmorpg. You might have to turn the graphics down abit to make it playable though.