Domain development phase is very important and can consume lots of time; domain is unlike a landed property that appreciates in values even when nothing is done on it. You must apply some developmental techniques on your virtual property before anything can happen to it.
Flippers must be skillful on how to develop his domains to make good money. Your domains should not just be left idle without making use of it; you must either park them or host them and write good contents on the sites on regular basis. But if they are parked, you should use your domain as signature under your contributions at forums after your quality contributions and also write articles that can systematically generate leads to the parked domains.
If the above activities are well carried out, then the age of your virtual properties now counts; the longer the age of your domain the more it will appreciates further in value. The flippers who do not believe in parking are losing money, because the traffic that comes to your parked domain means to buy one thing or others.
For your information there are domains that yield up to $150 on daily basis with good traffic techniques.
I personally believed on both techniques; by placing domain for sale and park with good domain broker and secondly, by hosting website or blog on the domains with consistent content placements on the blogs.
How to sell domain varies from one flipper to others but all I know is if your virtual gems are valuable enough, the buyer will get to you and negotiate for purchase. This is an area that requires patience, hard work and focus as it may not sell fast as expected.
The cost of your domain should be negotiated out between the bidder and the owner. But what matter most are the relevancy of the domain, age and traffic capacity of the domain should enable the two sides to reach a compromise on the selling price.
Further more, by checking the past sales of other related domains can also help to reach an agreement. You may not get buyer for some domain early, but the more the age of domain the more search engine pays attention to such domain and the more the domain will be more active and such will bring traffic and clicks. So it is not advisable to sell any of your domains hurriedly.
For you to develop your domains by hosting them, you need a web host that is cheap, reliable and specially established for domain flippers to enable you host for a reasonable price.