I installed a classifieds component on one of my sites just for a few days, about 300 categories and 300 dynamic URLs.

I now pulled off the component offline so that pages are gone.

Now I have 300 crawl errors 404 not found with Google but correct 404 headers.

Should I create 300 (301 redirects) or just let the pages fall in oblivion?

Additionally, I have another website with 5000 pages that I want to scrap and start a brand new site but with the same URL. Is it ok to leave all the other pages 404 to be forgoten by Google eventually?

Cheers!