In a private sales ecommerce site, users need to sign up/in before they can access the pages of website. So, even if a user tries to directly navigate to a product page, he is redirected to sign in. I am wondering then how does these sites manage their SEO, as it would imply google too can't crawl these pages, or do they completely ignore the SEO benefit of allowing google to crawl the product and catalogue pages?
Yes, google bot (and others) can't crawl the pages and the content. Showing the content only to bots (identified by their User-agent string would lead to Cloaking so is no option).
Some sites show previews of the content and allow the bots index the relevant keywords as most of the fun happens in page title, h1 heading, first paragraph and so on. Some sites with user generated content (help forums) with membership fees operated this way (show the question and the number of answers to tease you, but not the answers themself).
Some set up co-sites with simplified listings that link to the main site or submit parts of the content to relevant b2b sites that again link back.
Some identify their relevant keywords and put up public content with that to support their "hidden" content.