This is a common issue, and buyers should be aware of it.

You will see fantastic looking lenses being advertised at 400,500 even 1200mm focal lengths for your new digital SLR..

These are nearly always older obsolete lens that were designed for manual systems.  To get the focal length and to get them to fit a modern camera, they are normally a hodge podge of different components. Say an old Pentax 400mm lens, with an old pentax 2 x adapter, then they have fitted a Pentax to say Canon EOS Digital adapter.  Its suddently an 800mm ZOOM Lens for CAnon EOS DIGITAL CAMERAS

Problem?  Well the lenses generally dont focus properly or give the actual focal lengths referred to.  None of the automatic features will work (Shutter, focus etc) so you have to manually set everything.  This is not a bad thing as such (you get a better understanding of photographys that way) Added to that they are selling for a rediculous amount of money when you consider that individually you could pick all the items up for £20 to£30 at most. (and thats allowing for 3 seperate ebays)

Normally the sellers doesnt mention these facts, but in the few cases they do, its in type thats about 1/10th the size of the rest of the wording, and you dont catch on.

Also they will use pictures showing how great they are for long distance pictures.  Only problem I have with this is its the same picture that is used on every lens they ebay, despite them all being different makes and focal lengths.

Buyer beware....