Hello.
Some recommendations:
OPTION A
1) Develop websites with useful and relevant content on each domain. This basically means, treat each domain like an independent business and start working it.
2) Related to Step 1, market each domain.
3) Sell the business/domain.
OPTION B
1) Develop a landing page for the domain.
1.a) OPTIONALLY: Include a generic counter if you want to show how much traffic the site is generating (not a good idea if it is a slow domain).
1.b) OPTIONALLY: Include information related to current SEO data specific to the domain.
2) Sell the domain.
OPTION C
1) Establish yourself as a domain broker by creating your own business for this, complete with your own domain (which you are not selling ... yet).
2) Redirect all of your domains to your brokerage site.
2.a) You can redirect them blindly, as in just point them to your domainbroker.tld domain.
2.b) You can redirect them strategically, as in point them to their own sale page within your domain. For example, redirect grwngems.com to domainbroker.tld/grwngems-com.html where you can place your pitch, price, contact info, et cetera, specific to that domain.
2.c) Include "campaign" tracking information in the URL redirect. For example redirect to grwngems.com to domainbroker.tld/grwngems-com.html?src=grwngems.com or whatever you may use.
There are other options.
(Be careful providing a price for domains. You may undersell a property.)
