I will put it very simply, “You cannot purchase a genuine British title of nobility, with two exceptions, the title of a Scottish Laird (which is NOT a title of nobility) and a Scottish Feudal Baron (which IS). 

Scottish Feudal Baronies fetch very high prices (millions of pounds!), but Scottish lairdships cost only what the land costs to purchase.  Fortunately you can become Laird of a very small piece of land!!  Scottish Lairdships are dealt with in Wikipedia (Search for Laird in Wikipedia.org), where it states:   “Laird is a hereditary title for the owner of a landed estate in Scotland.  Though traditionally translated as "Lord", Laird is not a title of nobility. Unlike Lord of the Manor titles, the title of laird is a 'corporeal heriditament' (an inheritable property that has an explicit tie to the physical land), i.e. the title can not be held in gross, and can not be bought and sold without selling the physical land. Though the title Laird is not gender specific, in more recent times some female Lairds have opted to use the title Lady instead.”

However, many people choose to ignore this straightforward warning and still wish to buy a noble title.  Remember, it is perfectly legal to change your name, you can do that by deed poll to whatever you want (within reason); it is a simple, inexpensive process and you certainly do not need to pay anybody else to do it. However, whatever you decide to call yourself, the Deed Poll process will not permit you to take a noble title such as Sir, or Lord or Lady.

The current vendors of fake noble titles on eBay are able to advertise quite openly because the sale of novelty titles is quite legal - so long as they are described as such.  However you should be aware that what you are purchasing is a novelty and not a Noble Title such as the Queen can bestow.  You may even find that you can use such a Novelty Title to change your title from Mr to Lord on your bank account, drivers license and Passport - if that is what you want - and you are prepared to take the risk of being laughed at to your face.

However if the vendor misrepresents the facts, they step over the line between honest selling and a scam.  Any suggestion that you are being sold the "the legal documentation required under UK law to legally change your Title"  or a "Change of Title Deed" should warn you that you are being conned (There is no such thing as a Change of Title Deed, even in UK law - a simple search on Google will confirm this fact).  There are NO legal documents that will enable you to adopt a Noble Title.  If you feel the need to check that statement, search the internet for "Change of Title Deed".