We don't have to let them engage in unscrupulous practices. We have laws against lying in ads, for example, and consumer protection agencies exist in most countries with varying degrees of protection.
An example in many US states is the Uniform Act. The deceptive trade practices prohibited by the Uniform Act can be roughly subdivided into conduct involving either a) unfair or fraudulent business practice and b) untrue or misleading advertising. The Uniform Act contains a private remedy with attorneys fees for prevailing parties where the losing party "willfully engaged in the trade practice knowing it to be deceptive".
Such a legal setup shouldn't trigger the anti regulation people because it's merely a legal bias which, if the consumer pursues the matter civilly, gives legal redress against nefarious businesses.
Whether it works in practice is another matter. It would be easy to reverse the bias by ruling in court that consumers should be knowledgeable about misleading claims and the legal specifics of vague advertising terms.
I agree with the gist of your post as a whole - if a company knowing withholds or lies about relevant information in a transaction, then a customer which has been harmed by that is perfectly valid in bringing someone to suit and being paid for damages. Civil torts, along with other forms of legal harm to the business (like organizing boycotts, spreading information through word-of-mouth, etc), are perfectly valid and also ways in which individuals can discourage bad business practices. Organizations like J.D. Power & Associates and Angie's List exist solely for this purpose - to give consumers information on businesses to see if they want to engage with them or not. And if harm has been done, a civil case is completely justified.
However, regardless of that, I still think one should be cautious when engaging in transactions. This is like the rule to "read through everything on a document before you sign your name on it", and things along those lines. It's more just along the lines of covering yourself and being diligent and thorough in what you do.
Because it obviously still happens - despite existing regulations - that businesses and individuals false advertise, fail to carry out services paid for, sell malfunctioning products, etc. I would argue that's just part of life - because there are people who try to take advantage, or circumvent the rules for their own benefit; we, as humans, have free will, and that includes the free will to act poorly toward others. Therefore taking the extra effort to make it harder for such people to take advantage of you just seems like a good idea in general.
I mean, that's what I would do. But sadly, unscrupulous people exist, and so we have to use caution when we engage in these kinds of transactions.