The 7% is a fixed fee so that should probably be included in the calculation, but servicing tyres and rego could be an addition to every car bought second hand, however there is always an added risk to buying cars from auctions and arguably you can do a better deal privately with cash in hand.
It's certainly a great car for the price, but its holding value better than an M3 or a C63 or nearly every other car made in that year at that price point. I usually take redbook with a grain of salt however their prices are Private Price Guide $80,500 - $88,100 Trade In Price Guide $71,800 - $79,400 and for the premium * Private Price Guide $82,700 - $90,400 * Trade In Price Guide $73,800 - $81,500 * Average Km 50,000 - 90,000
I guess like with anything to anything the value of the car is what a person is willing to pay for it! I doubt many 15 year old cars can command what the BNR34's do these days, but there is always that bang for buck element with these cars