That would be fantastic. I don't think people in general would stand for all these restrictions on their speech and property rights if they had to explicitly agree to them each time they bought something.
It depends how different you imagine society being. If all we did was get rid of the IP laws but otherwise left things the same, people would agree to them without even reading them much like they do today with click-through EULAs.
For instance, buying decongestants at the drug store is such a hassle (thanks to anti-meth laws) that I find myself discouraged from doing it, even though all I have to do is go to the counter, sign a form, and show my ID. I could foresee a lot of customers choosing not to buy CDs and movie tickets if buying them meant voluntarily agreeing to pay thousands of dollars in fines and/or serve jail time if they used their knowledge of that music/film in the wrong way thereafter.
It's only such a hassle because the government requires it to be a hassle. If there were huge commercial incentives to make it as easy as possible and no laws requiring it to be difficult, it could be made trivially easy to do. (Like having a library card. If you had to do it to go to a movie, you would.)
Note that the terms could be much worse than the ones in the United States. With IP terms set by law, the terms are a balance between creators and consumers and include things like fair use. With IP terms set by contract, the terms are much more in the control of the creators and likely would include much more restricted fair use rights. This is why Microsoft uses EULAs.
Considering the difference in political clout between creators and consumers, I don't think there'd be much difference. There's basically no one looking out for the interests of consumers or defending fair use anymore. The "limited time" of a copyright term has gotten longer and longer, and been extended retroactively with no reason not to expect future extensions, such that it may as well be unlimited. The perceived purpose of copyright has shifted in many eyes, from giving authors an economic incentive for future work, to instead giving them their morally deserved rewards for past work. Fair use as we know it today is good for critics and parodists but has almost no value to consumers.
I largely agree with everything you say about how IP laws actually operate. Libertarians should be reasonably convinced that market-based IP agreements arranged by contracts will work out better.
Really the only live issue, other than Libertarians who don't understand the consequences of their own positions, is enforceability against third parties. As I've argued, Libertarianism doesn't work if third parties are free to ignore the consequences of other people's contracts. (The only reason I can't steal your TV from you is because I am not free to ignore the right to that TV that you acquired from that contract. If I was free to ignore that contract on the grounds that I wasn't a party to it, you would have no recourse against me when I stole your TV. I would not be required to accept that it was yours.)