How can I do SEO for Free classified website?



Doing SEO for a Free Classifieds website can be tricky because it's harder to control the amount of links on each page and the type of content which can fluctuate.
Do some serious marketing via Social Media Channels so you can gain some traction.  You might also want to do some advertising on social media targeting categories that are prominent on your website. This might not sound like SEO but ultimately you want to gain traction. The more visitors come to your site, the greater the chance of getting links to your website and then the more signals being sent to Google.
Use Video marketing on YouTube for the main categories of your website. If you have a section for lawyers on your website, offer your best lawyer advertiser a chance to do a YouTube How To Video that you will feature on your channel. Do the same for each category from Real Estate, Mortgages, Flower Shops etc. Remember, Google owns YouTube.
Guest blogging might help as well. Also consider reaching out to local bloggers and see if they will be willing to write a review of your website.
SEO work strategy for a Classifieds Portal
1. On-Page .
Check your Title , Description , Keywords.
Use Keywords on Title , Description and H1-H6 Tags
Use Long Tail Keywords and Use LSI Keywords.
2. Off - Page
Directory Theme Based,
Bookmarking,
Content Posting,
Forum ,
Guest Posting ,
PR Posting,
Images Submission (Use Photoshop for change File info of Images,Alt Tags)
Video Submissions
Blog Commenting
(Comment on those sites. if they providing List of Classifieds Sites)
3. SMO
work on all Social Media.
Join Marketing Community or Buy and Sell Community on Social Media.
Use all the basic strategies of SEO such as
  1. Meta description
  2. Post Titles
  3. Content with focus key words
  4. Quality and informative content
  5. Headings inclunding keywords
  6. Images with alt tags
  7. Post tags with topic wise catogories
  8. Provide social media book marking
  9. Easy way to  social share on platforms like Facebook Twitter Google and tumblr
  10. On and whole have a good responsive design
By including the above points have a good structure of forum to make the people interact.  The more people interact the more reviews you get so that the more products you can publish on your classifieds sites.