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.
Hi guys, just a thought, you do not need to build traffic to the domain yourself, you can always buy expired domains, best to subscribe to a paid service like dropping.com (so you are the first to know) or a free one, expireddomains.net and in this way, you can get old domains that have traffic. You could use these domains for parking, getting advertising income or even to sell. I have noticed, that domains that were not hot in the past, can still be hot now, just use Google Trends to type in the keyword, and you will see its future projections. 
If you buy a domain from a 3rd party, the backlinks are valuable.  I always wait a few months before redirecting, so just set up a quick minisite until two page rank updates have passed and then go ahead and 301 redirect.  However, it seems that you'll get the link juice from the back links, the anchor text is ignored by Google.   Buying a domain at Godaddy TDName expired auctions, seem to not count as a dropped domain if you immediately set up a mini-site.

As far as flipping domains for a profit is concerned, there are certain domain names that are worth a lot of money. For instance, Hotel.com and Business.com have been reportedly sold for $11 million and $7 million respectively. Now even though coming across such golden domain names seems more like a long shot, you can still make a decent amount of money if you choose to buy and sell domains as a part-time business opportunity.
The easiest way to see that is to go to HREFs and look at the anchor text. It reveals right away whether or not the site has been spammed. You also want to see if it’s been SEO’d too much. Right? In this case, it’s not because it has a brand name here and some natural keywords, but if the number one anchor text was like, cell phone management system and then the next one was like cheap cell phone management system, you’d know that the anchor text was manipulated and probably overly SEO’d. So, what you want as a site that has a natural link profile.

When you are selling a domain name, you don’t want to suffer because of failed transactions or being tricked by defaulters. Safeguard your monetary dealings; the best way to do this is to take the help of a secure domain transfer and escrow service. The escrow system is highly beneficial because a buyer has to deposit money at the trusted source prior to purchase, and the dealing between a buyer and a seller takes place through a secure domain transfer and escrow service. This makes the transaction safer for both parties with the involvement of a neutral third party.
Another point of concern is that many people feel that squatting or registering domains is unethical. This is because many web developers have a hard time finding relevant domain names for their projects since most of them have already been registered. Many corporate giants also frown upon the domain flipping industry because many domain names relevant to them are already registered.
I don't necessarily agree with some that dropped domains don't have value for SEO purposes but either way one thing you can do is to check out our tool for finding auction domain names (the tool has a database of domains available buy from GoDaddy and SEDO and also give additional SEO criteria to look up for the domains like backlinks, Compete data, CPC, Google stats, Alexa stats, etc. and there is also an option to schedule auction domain alerts so that you can be notified via email if a domain name meeting criteria you specify gets put up for sale) or you could use our tool fo domains dropping soon that shows all domains dropping in the next 5 days that way you can be ready to snap them up right away.
Whether you can use an existing backlink profile for a new web project under the available domain depends on whether the link target of incoming links can be reconstructed. In order to analyze which content was provided under a domain in the past, you can find out on the Internet Archive Backlinks have no value if they lead to nowhere and this can even have a negative impact. They are possibly redirected to similar subpages or homepages using 301 redirect. In this case the most part of the link’s power is lost.
But in general, the best and easy way to sell a domain is to go with a marketplace. There is many popular marketplace dedicated to selling domains including Flippa and SellDom to sell your domain (full disclosure, I’m working on SellDom). The advantage is to leverage the popularity of that kinds of services in hope that someone will find your domain and buy/make an offer.
Many non-savvy internet users may not even be aware of the existence of other extensions. That said, because there are many new extensions popping up every now and then, you can often find amazing one word and 3-letter domain names with these new extensions that are otherwise almost impossible to find unregistered in the .com world. Additionally, these new extensions are becoming more and more familiar to users every day, hence it's almost certain that their value will continue to increase as time passes and wider adoption manifests.