What about option #4 - Redirect your existing domain to the old domain? I bought an old domain that is 100% relevant to my current domain but currently has with very little content. It did have more content fours years ago. The old domain is 13 years old, pr=3, while my current is 7 months old pr=1 and a decent amount of content. The old domain I purchased was not expired though and I do not know if this makes a difference. What are the pros and cons of option #4? Am I correct to think that option #1 would result in no benefit from the old domain's age value and if so why is it not listed as a con, a MAJOR one. Its hard to believe that a 301 using option 1 would give my existing domain 13 years credit but I'll take it if it does.
Yes, we port in all of the domains from NameJet, SnapNames, etc. There are a lot of great deals to be found. Some are absolute trash of course but if you can sift through them and put some time in (hopefully that is what we are trying to do with our tools is save time and give some value add with the SEO metrics, alerts, etc.) then you can find some great bargains.
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.

If you’re patient enough, you can discover at least four to five very good .COM domain names at attractive prices. However, you’ll need to carry out some research and find out the right places where they may be available. I’ve seen that such .COM deals can also be bagged from popular domain marketplaces like Flippa.com, or from the private portfolio portals of sellers, provided you employ the right strategies. You can contact the domain owners directly too. There’s no harm in emailing them directly to negotiate the prices.
When you backorder the domain it will say how long you have to place your bid in. Let’s say it says 25 days. If the domain is from one of Namejet’s partners then the auction will start in 2-3 days. If this is an expiring domain and Namejet has to catch it there is a chance they won’t catch it in which case it won’t go to auction at Namejet. If they do catch it then 2-3 days before the auction starts.
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”.