you can't stop it

Just hide the post and get over it. It happens to me, it happens to you, it happens to everyone.

Well why not have a warning or something?

Where do you draw the line though? Lots of times a seller will say one thing but after enough time passes, they give in.

True, but there are a lot of patient people out there waiting for some "good" offers on their items. I just think there should be some sort of notification or something.

The notification is right on the trade, if they don't read the post their post get's hidden by its owner either without warning or with "Could you please read the actual post". http://www.tf2outpost.com/trade/12346900 Here's a good example. Saying "I'm reporting you because you didn't read the post" doesn't help anyone, just ask them nicely to read it over, if they start some kind of spam war, report them.

I even put "READ DESCRIPTION BEFORE OFFERING!" and then clearly state I want top market prices for items I'm selling (if I'm selling "X" and it goes for 1-1.33, the price is 1.33) and still get people who offer low/lowball. It angered me at first but now I just accept it and hide the offer, if they won't take a moment to read my conditions, I simply won't take a moment to explain why their offer is adequate.
Still, I believe there should be some penalty for continued ignorance, not banning, but perhaps limiting luxuries available on TF2outpost. For example: (After 3 occurances) reduce the maximum number of trades they may have open from 10 to 5 or 5 to 3/increase time they can bump their trades from 30 minutes to an hour. This isn't for malicious reasons, but to merely deter (and as a result will reduce others from saying "READ THE ******* DESCRIPTION/other flames/spam.") Just a thought :)

To be honest, people not reading your notes is mostly your own fault. The more text you write in the description, the more likely people are to not read it.
Keep your notes short and use simple language, not all people have English as their first language and not everyone sees reading your description as extremely important.
Have the information in the description in order of decreasing importance, starting from most important to least.
Use proper spacing and perhaps bbcode to help highlight keywords/sentences.

Read? Wtf is that item?