got an app for that
If the tech level supports it, you could probably take a picture of an item and get a list of sellers and prices, and of course adds for similar products, products totally unrelated but with similar names, accessories for the product, used items currently available but you must bid now, links to the latest social news, possible dating partners, increase the size of your...
The average person has a pretty good clue as to the value of everyday items. Specialized items might require a related skill, like medic for which prescription drugs are most profitable. (Firefly episode) Being able to price paintings, sculptures and other such items might not be obvious without some art skill. Of course if you are in the projects vs a mansion you don't need to be a brainiac to know which has the more lucrative items.
On the other hand, I like to role play and if the character has a background that would support this ability, then the career thief shouldn't need to have a wide variety of skills such as art, electronics, steward, weapons skills, computers, comms, drive, and so on.
On the third hand (you didn't know I was a 4 handed alien?) it also seams that if two thieves with similar thieving skill were to appraise something, like a vehicle, the automotive enthusiast with drive 3 should be better at it than one with drive 0. I'd role play a thief with medic skill stealing from clinics and a thief with computer skill targeting someone they see use the latest greatest model in the coffee shop.
On the forth hand, similar to above, two thieves with similar thieving skill but one has high Int (observant of peoples culture and current trends) or high Edu (reading and educating themselves in a variety of topics), should be better at appraising than the one with lower attributes.
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