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.
Adding domains to our Premium Listings on our website before you sell will give them more exposure to the community. We’ll offer an appraisal for your domain to give you an estimated selling price, and you can adjust that price at any time. We also offer a comprehensive breakdown of our commission rates for all premium listings, so you can easily understand the process.
It's ranking high today! What could be the problem? NO. JUST STOP. Unless you know the entire history of a domain, you may be setting yourself up for failure before you begin. SEOs (and business owners) use a variety of tactics to get a site ranking high in search results. For some of these methods, we'll just call them "questionable". These methods could include everything from buying links, overuse of directory submissions (non-industry related), duplicate listings, poor quality backlinks, and guest blog comments.
If you buy a dropped domain from Snap or Namejet, the backlinks seem to be worthless for SEO.  They are valuable for traffic if it's targeted to your site.  Go ahead and 301 redirect into your site because it's the traffic from the back links that is worth something.  I use the Google URL builder to redirect these names so you can see the domain the traffic is coming from.
If you are feeling ambitious, you can link each of your domain names to its matching “buy it now” purchase page at one of the marketplaces. Another more sophisticated option would be to “park” your domain with one of the leading domain parking companies, such as DomainSponsor or SmartName and enable their built-in “for sale” message and contact mechanisms.
Domains are nonmaterial goods. When sold, the only transaction taking place is transfer of domain rights from one user to another. Barring any form of copyright or trademark violation, virtually any domain name can be bought or sold. Generic domain names, such as car.com or computer.com, are not legally protected and can therefore theoretically be reserved by anyone.
I OWNED DOMPERIGNON.ORG, incredible, but true.  Lost it, back in the days when internet in India was not a readily accessible utility. And I was a casual domainer.  You had to go to a cybercafe. Did that late in the day, towards the expiry of the domain. No chance of renewal. The domain is now with the owners of Dom Perignon and is redirected to domperignon.com! 
This is a little bit on the grey hat/black hat side of the spectrum. But the fact is building a blog network with expired domains flat out works. The only issue is getting those cream of the crop domains without going broke. This video will walk you through tested strategies and techniques you can use to find powerful, yet affordable, expired domains.
By default we show the following metrics - Age, Ahrefs Domain Rating, Moz Domain Authority, Moz Page Authority, Moz Trust, Majestic Seo Trust and Citation Flow, Alexa Rank, Moz Referring Domains, Majestic Seo Referring Domains, Open PageRank, SimilarWeb Rank, SEMrush Monthly Traffic and Domain Price. Apart from these there are around 40+ metrics that you can see by customizing columns

Acquiring what you believe may be a valuable domain name is the easiest part of the equation.  Finding the right  buyer is much more difficult. Using auction sites and forums is perhaps the best way to find qualified buyers that are interested in your domain, and using auctions will take some of the stress away from establishing the value of a domain name yourself.

So the keyword planner came up with 332 keywords related to ripple, most of which can be used to inspire a domain registration related to ripple. Let’s go ahead and get some more ideas. We already know that ripple is another cryptocurrency. Let’s head to NameBio and explore crypto-related domain sales. We’ll use bitcoin for this experiment. I’ll type in “bitcoin” in NameBio search, select “show all” and then sort by price by clicking on the price column. Here’s what it looks like:
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