Well somebody has to break this to you:
Nobody understands what the fuck you're on about and what you are trying to sell! I guarantee you this will never sell because it seems neither you or us have an idea what you're doing.
1. A 6 digit isn't 6 letters long. It is a number. It is IMPOSSIBLE for your SteamID to be letters.
A '6 digit' account referrs to the SteamID32 (aka the Steam ID) being 6 digits long; that means it is in between 100,000 and 999,999 (note the number has SIX DIGITS in it).
A SteamID looks like STEAM_0:1:121412 or a random number.
THIS is an example of a 3 digit account. Note his SteamID is STEAM_0:1:254. 254 is 3 DIGITS LONG.
SteamID is the number in which place your account was created. A SteamID that says STEAM_0:0:300000 likely means the account is one of the first 500000 ever made (the steamID isn't exact). The first million accounts (all 6digs) are ALL 12 years old and from 2003.
The SteamID is viewable EVERYONE. It can be viewed ingame by typing 'status' in console in Valve games. It can also be found out by looking up a profile on a website like SteamRep.
2. Do you mean CustomURL or account name? Its clear you are mistaking either of those two for a SteamID. In that case, please actually specify that. Because you can't change titles of threads I recommend deleting this and starting a new one telling people you are selling a "Unique CustomURL - 6 letter noun". I personally used to have de_dust2 for a customURL before that got locked.
On that note, I can tell you that normal 6 digit CustomURLs aren't extremely rare so in the most insane case I wouldn't expect any more than $15. People don't want short account login names too much either so you wouldn't get any more for those (or less most likely).