so it's very important to download scripts only from known and authorized websites (for instance: mircscripts or Scriptaz).

That's a bit vague. I'm aware of both websites as I pass by them occasionally but I would have thought that the very best place to get a script (or any download) is the author's own website.

I've been supplying mIRC related scripts and addons for about 4 1/2 years and in that time I've learned that while it's hard to gain trust of others, it's harder to keep it. I've never had any doubters that I know of and this is why I've never contributed to submission services.

Don't get me wrong, this post isn't to slam such sites, I'm just putting forward the point of view that the type of site has no bearing on whether it's content is trustworthy or not.