Tomb-Robbing. Many wealthy people are buried with valuables; no reason that such nice things should be wasted on the dead. In fact, this is a great lead-in to the life of an adventurer
Depending on the level of magic or vindictiveness of the wealthy, this can be an extremely high-risk job.
Grave-Robbing. Many common folk will be buried with not-so-nice things, but these can still be sold. This is not quite so high-risk but is a quick way to get a very, very unsavory reputation. The 'common folk' won't think much about a thief robbing Lord Pembrooke's tomb and taking his jeweled monocle but they'll burn the house of the thief that clipped off the Window Marsh's fingers to get her heirloom rings.
Material Components. There are any number of evil spellcasters that might need unsavory components. Baby's blood, hand of a hanged man, that sort of thing. High pay, medium risk (you're probably in more danger from your client than the law).
Child Slavery. Maybe the Guild also employs a 'Fagin', a man who gets kids to steal for him. (Also, a Fagin may be his own Guild, in a way; he may also be a rival that must be eliminated). Kids can also be used for cheap labor, especially in mines where their small size is an advantage. Halfling kids make wonderful sneaks; their tiny size lets them get into places in ways no-one ever thinks to check. This might be recruitment of homeless kids on the streets, or active kidnapping.
'De-greaser' (alt: de-greaseer). In a more magical world, a sorcerer or wizard employed by the Guild (or who does this work as a sideline) to muddy the psychic/magical 'impressions' on valuable or unique items so they can't be magically traced or divined. (Also generally depends on there being more numerous, selective and active Divination spells in use in the game). A competant de-greaser can not only muddy impressions, but remove them altogether. A top-of-the-line artist can put
other impressions on the item. Useful for framing someone else for the theft.
Marker. A man who changes the maker's marks on items. Many things, from furniture to swords, bear a maker's mark (generally so you can tell if you are buying the work of the Master or that of his apprentices/journeymen). Changing this mark can mean selling a thing for a much higher price, or concealing ir's origins.
Coin clippers and shavers. This is one of the things that gets you drawn and quartered, most places. Both involve taking small amounts of metal from many coins to cast new ones.
Corrupt Scribe. Depending on the level of civilization, people may need contracts or other written things. This is highly skilled forgery work (for patents of nobility, duplicating seals, etc.) This is also a line of work that gets you dead, dead, dead if they find you.
Prostitution. Even if prostitution is legal, even if the local social mores are very loose, there will still be a boundry of propriety. And there will be those who go beyond that boundry, sometimes far beyond. The Guild provides for those people. Pay is extremely good, but again if you get caught, it's likely death rather than imprisonment or banishing. Again, a likely place for a Guild wizard, either for illusions and such, or summonings.
