A mint '98 with 8000miles couldn't sell for 18k on eBay. I'd have no problem paying up to $22k for your car if it's as mint as it sounds. CD player is a cheap option to be found on ebay. The roadsters are a lot more common than the coupes so pricing is difficult. Yours only has the low mileage going for it.
KBB and Edmunds seem to be a thing of the past. NADA is where you want to check for the price though they will be significantly higher. Without looking my best guess is a 98 M in as good as condition as you mention will go for about $25,000. Low mileage is hard to come by. Still not a bad buy. Try bargaining down a little and good luck!
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