Have you actually gone to see the bike? Is it the right size?

I would not necessarily rule it out until I know more about how the fork and front wheel got replaced. Was the fork bent in an accident, or did the Seller replace the fork because he/she wanted a carbon upgrade?

What happened to the original front wheel?

Non-original parts are a sign that the bike could have been in a accident, and should be checked by a bike shop before you buy it, AND non-original parts usually lower the market value of the bike.

After checking the bike over to make sure that there's isn't any frame damage, I'd offer the Seller $200, and see what he says. I probably would not pay $300 for it with the non-matching front wheel, but if I like the bike in person, and it fits, I might be willing to pay more than my $200 first offer.

Take cash with you.