I don't know exactly what kind of titles are being advertised for sale in the group you mentioned, but there are basically two different situations nowadays in Europe:
- Countries that were formerly monarchies and are now republics in which titles of nobility are in general no longer legally recognized and no new titles are conferred. Depending on the country, titles may have been simply abolished and it is actually illegal to use them, or they can still be used, sometimes as part of one's family name, but they are no longer regulated by the state.
- Countries that are still monarchies and where titles of nobility (or equivalent) are still legally recognized and regulated by different legal instruments, e.g. primary legislation (acts of Parliament); royal decrees; letters patent; or royal warrants. In those countries, which include e.g. Belgium, the Netherlands, Spain and the United Kingdom, titles are either granted by grace of a Sovereign, or inherited from the previous holder, or acquired by birth, according to the rules of succession to titles laid out in the legal instruments that regulate them. Such titles cannot be (privately or publicly) sold or purchased then.
In addition to royal titles and titles of nobility (in countries that are monarchies), most countries, including republics, also award orders of merit in different grades. Such awards are typically personal and non-transferable (i.e. non-hereditary). In some monarchies, e.g. the United Kingdom, appointments to certain orders or to certain grades within orders also confer kinighthood/damehood, while other grades do not. In all cases, however, those "titles" cannot be sold or purchased either as, again, they can only be conferred by grace of a Sovereign.
So, most likely, if you saw an ad somewhere offering a title for sale, it is either a fake title or some kind of fraud.