According to a late 97 patch note, it should be possible to use the item identification skill on items being sold by a vendor. The relevant patch note is as follows:
Bolding mine. This corroborates the information provided in this thread as well.12/16/97 patch notes wrote:Armslore
- The full range of armslore messages should now be reported.
- It now reports "brand-new" if an item has not been damaged.
- This now works on items in a shopkeeper's sell window. So does item identification.
On the demo, if a magic item is sold to a vendor, the item will be labeled as "a magic *whatever*" when attempting to sell or repurchase the item (e.g. a magic ring). It is also possible to attempt to identify items in a shopkeeper's sale window using the skill, and the regular messages regarding item's properties will be displayed. It should be noted that on the demo, the entire message is displayed as one large message, and is viewable without the journal, although some of that may be due to the differences in clients between the demo and UOSA.
Currently, on live UOSA servers, if a player sells or attempts to purchase a magic item from a vendor, the item will only be displayed using the same name as non-magic items (e.g. "magic ring" will display only as "ring" and can only be told apart by its cost). A careful reading of the patch notes and this thread seems to indicate that this fix was never successfully put onto the server. However, when testing whether an item in a vendor's sell gump can be identified, a magic item was successfully identified while in the gump, and by looking at the journal, the properties of the magic item can be seen. It should be noted that if you do not have direct line of sight to the vendor, you can still make a purchase from that vendor, but you cannot identify anything in that vendor's sell gump, and will receive the message "Target cannot be seen."
Given the above information, it does look as if Derrick did indeed change it so you could identify items in gumps, and that this is well evidenced via the patch notes and demo, and that it seems to be working correctly on UOSA at the moment. If you are unable to identify items properly, you might need to submit a bug report with specific details of what you are doing so we can attempt to replicate the issue.