2nd Hand Bicycle

>I have bought about 4 second hand bikes in the past 5 years (they usually last a year before they get stolen).

I've been cycling all my life and never had a bike stolen...... my last one was a total bargain. Bought second hand for IR£65 in 1991, sadly died of old age about 2 years ago. Transport for £5.50 per year or about E7! You can't beat that.
But I do lock them carefully! Lock front wheel to frame to immovable object. Also some of the multistorey carparks have bike racks, which keeps them out of the rain too.
Twice I've had the wheels damaged, presumably by frustrated thieves.
 
not sure where you are located but I heard good reports about a yearly bicycle auction of stolen bikes with no identified owners at Kevin Street Garda Station in Dublin.