You don't need one, but most people only trade with those with a thread. And ya, they're kind of like shops, but you don't need to sign up for one. Just post it.
And since Smogon is a Competitive website, people only want competitive grade Pokemon. That means Pokemon with good abilities, natures, and IVs.
You'll also notice that most people here aren't as kind as they are on BMGf. Don't worry, it was like that when I first started too :0