I can't speak for the 'intended practice in 4e', but my opinion is that it depends on the focus of your campaign/group. If you want to focus on the storyline, combat, adventure, and other similar things, I would just do something along these lines:
DM: You find a Polished Ruby lying by itself in the chest. It about a 1.5 in inches long and throws brilliant red light across the walls when you hold it up to your torch. Would you like to take the 250 gold its worth or keep it?
This way the players still feel like they're getting cool stuff and not just money all the time, even though that's what it ends up being. They still have the option of taking the ruby itself in case they feel like they might need to bargain with goblins that have no love for gold but are quite interested in anything shiny.
On the other hand, if you're group focuses on commerce and item-dealing, you might want to make them actually find a place to sell it. In this case, you can give them a range of what it 'looks like' it might be worth and have them go somewhere to find how much they can actually get for it:
DM: You find a Polished Ruby that looks like it could be worth
anywhere from 175 to 500 gold pieces.
Player: Interesting, that's probably worth holding on to to get appraised somewhere.
(they bring it to their favorite shopkeeper to see how much he'll
actually pay for it)
Shopkeeper: Hmmm, looks interesting, but I actually have a few like it
already. I'll give you 200gp for it.
This is probably the most 'realistic', since they players can probably come up with an approximate value for it if they examine it, but they might not be close. Supply and demand can also fluctuate, so if you like that sort of thing and it plays a significant role in your campaign you could probably take advantage of the differences between apparent value, true value, and offered price to make for some more interesting situations. :D
On a related note, I personally am a fan of decreasing the value on objects whose primary purpose is to be sold, but then letting the players sell it for that entire value instead of at a cheaper price. This prevents situations like this:
DM: You can sell it for 200gp.
Player: How much is it worth?
DM: 250gp.
Player: darn, even though those merchants have to make money somehow I feel like I'm getting ripped off. Maybe I'll keep it and keep asking you until I find somewhere where I can sell it for 250gp.
DM: (thinking): [oh great, should I just give him the stupid 50gp extra so we don't have to spend any more time on this? ]
Hopefully, you get something more like this:
DM: You can sell it for 200gp.
Player: How much is it worth?
DM: 200gp.
Player: Cool, full value! I'll sell.
This way the player is happy they got their money's worth since they weren't going to get much more than 200gp for it anyway, and the DM and rest of the group are happy because you all can move on with the campaign. :D