here in my country there are a lot of sell-buy-type sites like ebay somewhat. Two of them are the most popular. I'm thinking in creating a website that when you list something to sell, it automaticly adds the advert to all the other websites.
I'm not planning to talk with the websites owners. So the question is: will the other website owners have the right to sue me, or the only thing they can do is to improve their websites so a script can't make posts automatically?
Your question was 'can I be sued', and the answer to that is unequivocally YES! You could be sued. People can sue you for basically anything. Whether or not it gets to court is a different matter.
However, as most people in this Q pointed out, they will most likely just block you. It's not worth the effort to take you to court. (Unless they find out that by violating their terms of service you made a whole bunch of money, in which case, they would probably do more than just block you).
Sounds like you already know you shouldn't be doing this. Why not request for them to offer an API or figure out a way where you can post on their site and it helps both of you? (i.e. if they charge a fee for their site and you are paying that fee, then why would they care? if they are selling adverts on their site and you are generating content for them, why would they care?)
I think before you get sued, you might get blocked. Some sites encourage 3rd party ads, and probably would give you an API where you can make the posting a lot easier. Some sites , like craigslist.org make it very hard to do this, and police it.
They dont necessary sue you, but block your ip's and prohibit you from posting. Every site will have its own terms and conditions for you to review.
If its against their terms of service, they will probably just block you with captchas or putting requests from your IP on a blacklist. If you were making money off it it, its possible they'd at least try to sue you, but you'd have to be making enough that they'd turn a profit after lawyers fees.
In general in the escalation goes
Block (if possible) -> Takedown Notice -> Legal Threat -> Legal Action
If you're this certain you'll get either blocked or sued, it could be valuable to find a way to work with agencies by building out a prototype site and getting them to get you API access. If you get buy-in from the beginning, you can help each other out and it adds a barrier to entry for possible competitors.