Should I buy unity pro if
- I am making some scripts/assets/prefabs and sell them?
- I am making non-game applications in unity-free version and also sell them?
You can sell scripts without the pro license, the only thing pro gives you are extra rendering and other features and no logo on start up
you can even sell games with the free version.
You could always sell a few scripts, and eventually have enough to get Unity pro.
your answers are confusing to me. I would like to hear an answer from some official here or get a reference to user agreement.
the download page has the EULA
But to clarify what they meant:
Unity Free allows you to create games and sell them, create prefabs and sell them, create scripts and sell them or in simply: you can use it for commercial work.
What mikesgames wanted to go at likely is that once you or the incorporated entity earns more than $100’000 per year, you MUST buy Pro, you are no longer allowed to work with the free unity. This is explained at the bottom of the license comparision page
wow this is kinda cool!
why unity is so generous?
I mean I can understand selling code or prefabs even if it were illegal, because it is just not worth they layers effort to tracck. But for the rest I serious thought the free version is just for evaluation only.
If I would have to guess then to allow you using Unity to its best, have success and upgrade to Pro, making you a success story for Unity in two ways 
I think the idea is that they can get a ton of people using the software if it’s free, and many of those people will later buy the pro version. It helps them compete extremely well with the other free/cheap engines out there. Anyone who wants to develop for iphone, android, or any non-PC/Mac environment will also be purchasing a license.
Having Unity free for commercial use can also help push more browser plugin installs. It’s definitely a win for everyone. Though I have to say that I’m quite excited to get my copy of Unity Pro in a couple of days and play with all of the new bells and whistles! 
Why are they so generous? Cos they’re awesome that’s why
Seriously though, it’s also a great business model. Some people seem to view that as a negative thing, that it’s strictly business, but it doesn’t change the fact that we’re still getting it free (and even better for commercial use). And it’s a win-win situation for everyone. Giving Unity for free allows people to make games for free. If they do make money off their games, they’ll likely buy Pro, and even if they don’t Unity still gets another user. UT gets more users and people buying pro and we get Unity for free for commercial use. Hell you could even make your game and sell it, get pro and then add the post processing effects as an update afterwards.
Granted you get a lot of people who download it just to have it and aren’t really serious making games but UT wouldn’t have made money off them anyways.
Me, I’ve got Indie and couldn’t be happier but you can bet that once I’ve got the money I’ll be buying for pro. In the meantime, I can continue to develop my game and by the time by the time it’s done or even before, I should have enough to get pro.
Keep in mind the limitations of Unity Free can be a factor even outside the gaming world. For example, not too long ago someone sought a Unity programmer to make room designer software with lights the user could move and turn on or off. Without realtime shadows, you wouldn’t be competitive. Then there’s iPhone support, support for third party libraries (unmanaged), support for good water and glow effects, etc.
My story. I was contracting Flash for some big video agency SOB’'s and chasing a 7k USD payment to starvation led me to bitching at the coffee shop about Flash and how I have started to hate it after 12 years of developing in it. I mentioned I am a 3D tech director but there ain’t much “fun” work in that field in Atlanta and I would do games but there is no Mac engine … He told me about Unity Free and I thought…this will suck just like every other game engine I tried and couldn’t get anywhere quickly with. I dl-ed it and in minutes I had things spinning and towers of blocks crashing into the ground plane… Woot! No render waiting! Rigidbody dynamics! A week later I had a walk around terrain, a carnival room with phones that rang and you picked them up and they spoke messages, doors opened when close and I was doing pickups of “molecules”. I sent it to a fellow I know and said to look at this game engine. I continued developing some game frameworks and showing them to him. Five months later he funded this company, made me the CEO and him President, got me a really nice two story loft as a live at office, gave me free run of the credit card, put tens of thousands in the account, bought six computers, three ipads, two iPhone, one iPod, two seats of C4D, two seats of Unity Pro and iPhone Pro, Assets server , two seats of ZBrush, two seats of Coat3D and an R&D license for SILVIA, as well as a round of mixing boards, firewire MIDI interfaces, expensive headphones…oh…and a new 4WD vehicle!
Without Unity Free I would have never taken the plunge due to clowns like GG giving me bad impressions of game engines that aren’t worth a million bucks…which I figured I would never get my mitts on anyways… As well I found this app when I was being ripped off and dead flat broke. Now I am a happy camper.
BTH