If you know how to get websites to page one, why are you not marketing that fact to potential clients and consumers or would-be domain purchasers? Trying to get a keyword driven domain to rank high and sell it off for a profit isn't a good investment, either time wise or for the long-term success of your company. Instead, use sites that you have already ranked high as an example of how awesome you are and sign them up for a monthly fee, rather than trying to sell them a "make money now" domain.
Just like a property listing, except much simpler, domain marketplaces are basically massive lists of domain names that are up for sale. The process of using them is simple. Buy a domain and park it, then list your domain on the marketplace for a price you’re willing to let it go for. Once the domain is sold, the marketplace takes a cut and then passes on the remaining funds to you.
Surely if you purchase a domain (domain1.com) and redirect it to your existing domain (domain2.com) you expect to loose all ranking for domain1.com. If 301's are used then the objective is simply to pass on authority, not dominate the search results with 2 domains. If you want to keep domain2.com in the index then you would take Rebecca's option 2 or 3.
Instead of focusing on one domain name to sell quickly, try selling large volumes of domain names over longer periods of time. While this may require a more substantial investment, it will also return a greater profit in the long term. Remember, if you follow the basic principles of domain name speculation, there is good chance that your domain names will sell eventually.
So how do you develop that instinct? NameBio maintains a database of over 500,000 historical domain sales (as of writing this post). They have interesting filtering features by which you can narrow down domains by price range, date sold, keywords and more. Simply sifting through the listings on NameBio long enough will quickly develop your domain appraisal “instinct”.