Second hand Scott Comp racing/ Should I buy?

Since 2001, over 90,000 cyclist's have joined Cycling Forums to discuss topics from general cycling to equipment, training, racing and travel or vacation destinations (especially in europe during the tour de france). We also feature an great deals in our online store, 100's of articles, classifieds and product reviews.

Can anyone advise on buying the a second hand Scott Comp Racing 2004?

Seems the seller is offering for about 50% of original price.

Only info I have is he has done 3,500km on it.

What should I be asking seller to know whether this is a good deal or not?

Is 50% discount on a 3 year old model with this milage a standard mark down? What sort of costs would I have to replace parts at this stage?

Thanks guys for any advice.