I recommend reading also Money making for the Noobs on Robin Hoods' site.

Where to sell junk

Nearly all you can get from NPCs or shops is that - junk, virtually useless for players. There are two kinds of places where you can sell low-end equipment - general stores and fences.

General stores

There are lot of stores in Ankh-Morpork and other cities - just check any map and you'll find one nearby. They buy about everything, except stolen or just really worthless and damaged items. Sometimes general store is full and can't buy any more - just wait a couple of minutes or try another store. Shops give much less for damaged items, so it can be good idea to fix it first.

Fences

Fences are people that buy stolen items. They always have specialisation, usually clothes or jewelry. Fence will pay more for items of their specialization. Fences in AM are Jim and Sol. Both are in Thieves' Guild (a little east from Mended Drum, check a map). Jim is always in a room just north from entrance and he specializes in clothing. Sol is wandering somewhere around Jim and specializes in jewelry. Use syntax "fence /items/ to /Sol or Jim/" and then "nod /Sol or Jim/" or "say yes to /Sol or Jim/"

You can find complete list of fences on thieves' guild page

Where to sell useful stuff

Mostly player shops. What you can sell is mostly depend on your guild skills. Wizards sell spell scrolls, witches sell teas and crystals, priests sell charged rods. Different shops specialise on different items, as you can see by looking at shop inventory. Ask shop owner about further details.

Scavenging

Even without any skills you can get some money by scavenging. There are two kinds of it - "search" and "recover". By performing "search" on streets of city you can find some money or item you can sell. Usually you find clothes in horrible condition - fix them by Gapp's low altar or other means before selling - you will get much better price that way.

Other way is "recover". Most high-level players don't bother to loot everything from things they kill. You can "recover" corpses they bury and see if they have something sellable. Just don't forget to bury them again after that. Also you can check Ankh river under back Drum door ("jump down" from door). Corpses of people killed in Drum fights are thrown there and sometimes "authors" of kills don't bother to go and bury them.

Banditry

All-time favorite method of getting gold in MUDs - killing stuff for loot. It's fun, it gives XP and TMs, and it's comparably safe if you pock your targets wisely . Use command "consider" to get estimate of how hard would be killing of particular NPC. Check How To Die for info about some specifically dangerous NPCs. "Look" at him/her to get estimate of potential loot. And be ready to flee if things go nasty. Good targetsin AM are thieves, lawyers and such. If you aren't going to run to general store after every fight or two (that could be an option, if there is one store nearby), don't loot everything on corpse. Loot just money and things that looks valuable enough for their weigh, like jewelry and silk things.

Stealing

Check thieves' guild page about stealing

Stealing is a good skill to get (or save) some quick bucks. Skill covert.manipulation.stealing is used for both pickpocketing and shoplifting. If you are not a thief, stealing will be a little harder, as you don't have this skill as primary, can't steal in Ankh-Morpork (well, you can technically, but then you will soon wish you couldn't) and can't predict chances of shoplifting.

Both stealing and shoplifting are nearly risk-free. You will have nearly no serious consequences for shoplifting, and if you'll fail a stealing, you will be attacked, but you can just run away in that case.

As non-thief you can steal items in inventory and money, and snatch items from hands. With 200+ bonus you will have good chances of stealing money from about anyone.

You can try to shoplift about anything from shops.

Difficulty of stealing is dependent on item cost and weight (the more, the harder) and shop base security level (usually higher in bigger cities, except Ankh-Morpork). Each time you (or anyone else) is stealing or trying to steal from shop, shop's security level is raised slightly.

Overall, with moderate skill you can steal reliably only light and inexpensive (~10$) items. Both jewelry and clothing are usually not heavy and can be sold to appropriate fences, so that are good stealing targets.

Bes Pelagric family missions

Agatean families pay about 10-20 rhinu amd 20k xp for mission. Considering it takes about 3 minutes to do it with some practice, it's not that little. Main drawback is that you can do a mission only once in half an hour, so this method is suited mainly for idlers.

Guild specific methods

Only a brief list - consult your guild page for details

Thieves

Stealing, obviously. Everyone can steal, but as a thief you a have stealing skills as primaries, thief-only commands and Ankh-Morpork as a playground.

Wizards

Scribing scrolls. Non-wizards can cast spell from scroll if they have a skills, but scrolls are one-use only, so they are in constant demand.

Witches

Brewing teas and making crystals.  Most player shops that stock tea will buy healing tea with a BWB (brewed with bonus) of 300+, and some player shops well sell 'newbie brew' at lower bonuses to give young witches a revenue stream.  Witches can also enchant items up to 'a steady octarine glow' (level 5) for a fee.  Although high-level players will want high enchantment from a wizard (up to radiant) on their items, levels of enchantment 1-5 can be useful for low to mid-level players.

Priests

Rod charging, especially if you have longsight ritual. As a Pishite - donations for resurresting, restoring and pet raising.

Assassins

Inhumes are the assassin's 'day job'.  Covert skills allow assassins to steal legally outside of Ankh-Morpork.

Warriors

Banditry, killing and looting.