I see people entering the market and making basic startup mistakes. This is normal and is part of the learning process but below are a list of ideas I've formulated in the 16ish years I've been buying and selling domains.
1) have an idea what you can do with a domain when you register it. If you think it has resale value think of your target buyers. If you think it can be developed and monetised know in what area and how you're going to go about it. If you cannot think of a reason *don't* register it. 'Brandable' is a meaningless term applied to domains with no meaning - these types of sales are incredibly rare.
2) If you are selling via a forum remember estibot or any other appraisal tools are very seldomly an indication of its value. A domain is worth what a buyer will spend and a seller will accept. Quoting values from automated tools suggests that you have no idea what it's worth to you and therefore your expectations will be unrealistically high - such quotes are a red flag to serious experienced resellers.
3) You will get higher prices in an enduser sale. This seems pretty obvious but there are many people with freshly registered domains who attempt to sell on forums for enduser prices. You are better approaching potential buyers directly and individually. Although you are at a slight disadvantage as you have made the first move you will likely sell for higher than resell value - which is what forums are for. The rule for value is usually : enduser approaching you > you approaching enduser > forum. Of course forum is liquidity and you can quicker sell a domain.
4) If you are selling a developed website bear in mind people will really only be interested in the traffic and revenue. Unless the domain is outstanding you should value the sale at a multiple of the revenue and any additional costs you have spent getting it succesful. Generic constant traffic commands a higher price than trends in quick moving markets.
5) Understand what goes into a domain to give it value if it doesn't have immediate obvious resell advantages. The argument 'ebay have a made-up word and their domain is worth millions' doesn't cut it when you can take any made-up word, spend MILLIONS marketing it, and give it value if it provides a needed or wanted service. You likely haven't spent that on a fresh registration from two days ago.
6) Constantly adapt and evolve. What worked a decade ago doesn't work anymore. What works last year most likely is not the best way to do things now. Keep abreast of the market and its movement.
7) Stick to .coms, established old extensions, or in some cases ccTlds (country codes relevant to the market). Yes there have been a couple of good sales in other extensions such as .top, .club, .banana etc but these are tiny compared to the amount of speculators. You have more chance of selling a worse .com than an excellent gtld.
8) If nobody is approaching you for a domain you hold within a year it is a good indication that it's not as sought after as you wish to believe. Don't be afraid to let bad domains drop. There is no harm in trying and failing - most of your domains will fail it is normal. You want to hit on the good ones to cover the bad ones. As has been proven if people have deep enough pockets they can register a large portfolio with the knowledge that statistically some will sell and at least cover the renewal fees of the whole thing.
9) Don't be afraid to spend money to make money. The days of fresh registrations immediately selling for lots of money have gone. Yes there are 'lottery' type sales now and then but you cannot base a serious business way of lifestyle on winning the lottery at some point. It is always worth buying domains with established routes to monetisation for a price you are comfortable with and the price a seller wants to sell for. Work out how much the domain will earn you or, if buying for resale, have a minimum level of profit you are willing to accept. If a resell domain is 'worth all of it's money' then beyond vanity you will not have much to work with. However as buyer and seller expectations often differ you can sometimes buy domains which you value higher than the seller does.
10) Accept there is a standard 'market' value of certain types of domains. You may not have to agree with it but 3 letters, 4 letters (com) have a minimum 'token' price. You will always be guaranteed to get that price in a reseller market. Keep abreast of all domain sales and notice general market trends and standard values. Ignore some of the lottery wins you will see. I have bought and sold many 3 letter .coms in the past and simply trading on the profit in a reseller to approached enduser value makes up for not selling 'this-domain-is-the-greatest.com' when similar rubbish has sold once the month before for umpteen million dollars.
11) Don't be buying at enduser prices to sell to resellers. Likewise when posting a domain for sale on this forum remember that you are speaking to resellers and enduser pricing will not apply here.
12) Be honest in the reseller market. It is often noted when people try to sell a fresh registration for 6 figures and, when value is mentioned and that it is too high, claim they have already had offers for a naive figure. We know that isn't true - we are not silly. That just raises warning flags to serious buyers about dealing with you in the future. Also don't claim that you are selling domains because you don't have time to develop them and if they're not bought by xx date you will develop them yourself. Never indirectly threaten people who may be your customers especially when they can see through the bs they have seen so often. Reputation in the domain industry goes MUCH further than a few dollars. Similarly when asking for appraisals on a domain expect that opinions will differ. There is absolutely no point in getting upset on a valuation opinion. Remember opinions are free and largely subjective.
13) Learn what 'premium' really means. A lot of domains are touted as 'premium' when they clearly aren't. This means either that the seller doesn't know that (and needs to learn more before he makes money) or does know and is trying to pull the wool over the eyes of the buyers who know better.
Feel free to add any other observations.
Really Great Ideas and Suggestion I have got while reading this Awesome Post, hope others also like your Ideas ...