I've seen Ebay littered with loads of these:

http://www.ebay.co.uk/itm/1920s-adv...5ae5ab24a8

My first question would be are these old advertisements actually genuine and does anyone buy them? Some of them are very deserible but I don't know whether they'd be worth the trouble to frame.

Any insite into these would be appreciated.

Unfortunately ads clipped out of old mags seem to put off the purists. They are seldom ever worth more than you would pay for them.
Having said that if you like them its a cheap way to put up ads for low money. But it may be just as worthwhile to scour charity shops, secondhand bookshops and boot fairs for the original mags to look at and keep or clip them out yourself.