Well, the two biggest areas to consider are marketing and security. Marketing is by far more fun and basically consists of positioning your account to sell. You can try things like forums or craigslist (ebay bans these sales), or you can try a site dedicated solely to the buying and selling of mmo accounts. The latter falls into two categories: individual and corporate. Corporate ones are pretty simple. They give you a lowball offer, but are pretty much guaranteed to be legit. For your account I wouldn’t expect much more than 100 dollars from one of these sites. Max you might get is $150 if you are lucky. To put it in perspective they offered me around $250 for a pally in 6/8 tier 6 and multiple BT/hyjal offsets (both ret and prot) as well as multiple s3 PVP sets – not to mention a second lvl 70 as well.
The other option is player based sites. I use (and recommend) mmobay.net (NOT mmobay.com). This site helps keep out the riffraff as it does cost a small fee to list an account (there are various exposure options, but you can get a few bells and whistles for under $10 – worth it if you plan on selling for $300+). Naturally this kind of site does run some risk getting scammed (I will talk about this more in the security section). In order to make an effective listing, start first with the title. Generally no one cares about your alts. Try to get as much info about your main as you can. An effective title for you might be “80 frost Deathknight X/X t7.5, X/X deadly.” Your screenshot is also key. Put on your best gear (preferably set gear) unsheath your weapon and zoom in so you take up most of the space. You will also post more pictures (in other sets of gear and of alts) in your listing, but these won’t be displayed unless someone clicks your listing.
Basically, you should have several sections to your listing. The first should be general info such as race, faction, spec, and professions. Next you will want a full listing of gear. Make sure that you mention what slot each piece is, and also change the text color to match the quality of the item (i.e. purple text if it is epic). Lastly, you will want a section of additional information (reputations, important achievements, liquid assets, epic flying, and info about your alts). This may seem a bit tedious but it can pay off. The best advice is just to look at some of the highest selling auctions and get ideas from them.
Security: FOR GODS SAKE DON’T PUT YOUR CHARACTER NAME IN YOUR LISTING. That is rule #1. Don’t have it anywhere on your auction. Prospective bidders can send you private messages and you can tell it to them there.
Next, you will need a system of payment. Paypal is preffered. You will need an upgraded account to recieve payments over $50 i think, however this is free to do (paypal may take a very small commission from the sale) The one problem with paypal is that buyers can withdraw the transaction at anytime before it clears. This means you must be very clear that NO account information will be given away until it clears. Also you MUST talk to the person on the phone or over vent. This doesn’t really do much, but it is just one more obstacle that scammers won’t really want to deal with. My buyer was australian and seemed quite legit, so I actually released the account info before the payment had cleared and was fine. I still retained the secret q/a (this is your backup btw – this will allow you to get your account back, so NEVER give this out unless the payment is cleared and has been moved from paypal to a private bank account) untill after payment had cleared. There are various other security precautions (such as a mutual $1 paypal exchange) that you can consider, so check around other auctions and see what they do. If they seem sketchy, they probably are.
Anyway, hope this helps.
P.S. Now isn’t really the best time to be selling (since blizz is in the middle of expanding WOTLK and all that gear will be garbage in a few months), so I have doubts you can get $400 for it but good luck.