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.
Review Web Archive’s versions of the website: Examine the website’s history and how it looked like using the Web Archive. What was it about? What did it offer? This is useful for a variety of different reasons. It could inspire you as to who a good potential buyer might be (by analyzing the website’s content), and it could also signal some red flags in case the domain was being used for anything shady.
Just because someone spent five or six figure amount on a domain name doesn’t automatically imply that he/she’ll spend the same or at least a few thousand dollars on a similar name too. Basing your domain flipping strategy entirely on what others have just bought may not be as useful as you may think. Every domain name has a story behind it and its purchase/sale is usually as unique as that story.
In theory, you could technically acquire a domain for $10,000 and be able to sell it for $100,000. You could also acquire a domain for $10 and be able to sell it for $100,000, $10,000… or $100, you get the picture. Because of this, it’s hard to recommend a perfect starting budget because your strategy, experience, and the pace at which you learn will hugely affect your buying and selling decisions.