Examples: H, 4, S, 8
2 - Skill names: Also extremely rare due to the limited amount of skills. There are only 25 skill names able to be taken, since Dungeoneering is too long, and even less available due to bans and inactivity. Again, extremely rare.
Examples: Mining, Fishing, Slayer, Strength
3 - Words: These are common everyday words that you'd hear in everyday speech. This does NOT include names with suffixes such as -ed -s -ing etc. Generally the shorter they are, the rarer they are.
Examples: Hi, Fat, Fire, Peace, Legend, Monster.
4 - Two (2) character Names: These are names that only have two characters in them, any combination of letters and numbers. The rarity of these dropped significantly when hundreds were released into the game during the name release, but they are still worth a fair bit.
NOTE: Albeit being more rare (numerically), letter-digit combos seem to be worth less than letter-letter combos.
Letter-Letter examples: Nc, Sa, Le, Bp
Letter-Digit examples: d3, 8a, v4, f6
5 - Two (2) Digit Names: These are digit-digit names, and are considered rare due to there only being 100 of them in-game, and even less due to bans and inactivity. Hard to get your hands on one of these, so best of luck.
Examples: 17, 48, 79, 37
6 - Certain in-game Items/characters: Not all of them in-game items and characters are rare, but some of them are. Among the list would be gods, extremely common single worded items, very common characters, etc.
Examples: Zamorak, Bandos, Elf, Gnome, Glory, Fury, Vorago, Sword
7 - Real Life Names: To the right person who has this name, these names are worth a LOT. Many people want their real life name as their RuneScape display name, so the more common ones can fetch high amounts.
Examples: Kyle, Sarah, Ryan, Rebecca
8 - Days/Months/Holidays: These are all rare in varying degrees. And no, unofficial holidays such as "Donut Day" or something are not considered rare. Mainstream holidays.
Examples: Sunday, August, Christmas, Easter
9 - Animals: These are always coveted by animal lovers and people who like their favorite animal.
Examples: Shark, Eagle, Tiger, Fox
10 - Certain Anime Characters: Many people like to sit down and watch a good anime show every now and then, but you'll run into those who are diehard fans of certain ones, and would pay a lot for certain anime names.
Examples: Goku, Shin, Yagami, Naruto, Tweety
11 - Big companies/social media: When I saw big companies, I mean massive ones that everyone knows what they are. General rule of thumb, if you have to explain it, it isn't rare.
Examples: Disney, Google, Twitter, Skype
12 - Certain cars/manufacturers: Generally only the manufacturers are considered rare; however, if you have a very common car, Prius, or fancy car name, Gallardo, you may be able to get something for it.
Examples: Honda, Toyota, Prius, Gallardo
13 - Common Words in Other Languages: Please don't try to say something like 'silla' is a common spanish word. I'm talking words that most people will know.
Examples: Si, Hola
Yes I copy pasted this, because clearly selling your original RSN you made when you were 8 just floods the forum with names no one would ever buy xD