Police, in all likelyhood would not be interested, they would say its a civil matter, its goods not as described rather than theft.
If you get hold of his home address one thing to consider would be to get a quote from a shop (preferably 2 shops) to bring it to the condition it was advertised as then ask him, in writing, either for the money back (and you'll return the marker) or the money for the repairs as quoted by the shops. Make it clear in the letter that if he doe not chose one of the options and write to you with his response in 14 days you'll have to resort to issuing a court claim. Keep a copy of the letter for the next stage. Make sure you've a copy of his original ad, him agreeing to sell it for the price you paid etc.
Have a poke about on "moneyclaimsonline" if you decide to sue, for fairnesses sake you should be suing either for the amount you've been quoted by a shop to return it to the state advertised or the price you paid, whichever is lower, court fee is £25 for a claim under £300 which gets added to what he owes. If he ignores it he ends up with a CCJ against him which is pretty much his credit history stuffed for the next 5 years. A judgement is no assurance that he'll pay you but it is the only way to get a CCJ off your record and if you want you can send bailiffs after him once you've got a judgement (again always a chance they won't recover anything).
