It's certainly not the going rate for M113 engined SLs being sold privately. I think I've chased up every decent car that's been advertised anywhere - the cheapest you'll get a one with average mileage and a glass roof is £16.5k. They're selling too unless at silly dealer prices of £20k+.
The problem with SLs is that there are a lot around that have been mistreated
Glass's Guide reads £13300 for 98 S reg with 69000 miles or £14100 with mileage correction. If the car is 100% straight in every way then £15000 ONO sounds cheap to me. Yes you will probably buy it for a bit less at auction but you cant give it a proper once over & a test drive at least here you can and possibly pay not much more then at auction where you buy with warts and all.
I have bought a few cars in the past from the papers which have been perfectly straight and paid less then or same as what you would pay at auction.