On D&D 3.X and Pathfinder, you can't really crit on Skill Checks. A 20 is just a 20, and a 1 is just a 1. There's no auto-fail and auto-win associated with Skill Checks.
I, for one, simply love the moments of "awesomeness" that rolling a Natural 20 brings to the table. So, even if, by the rules, rolling a 20 on a Skill Check does nothing special, I bend the rules a bit so cool things can happen. I elaborated a few rulesets on this:
Ruleset 1 - The Boost: When a Player rolls a 20 on a skill check, add a +10 to his check and make him roll again! This will make rolling a 20 almost a auto-success.
Ruleset 2 - The Boom: Similar to the Boost. When a player rolls a 20 on a skill check, "explode" the dice: make him roll again and add the results. (First roll was a 20, second roll was a 15 - Total roll: 35!)
Ruleset 3 - The Token: When a character rolls a 20 on a Skill Check, he gains a Skill Token for that Skill. Later, he can spend that token to reroll a Failed Skill check.
- Example: Bob the Bard Rolls a 20 for a Bluff Check. He gains a Bluff Token. Next time he fails on a Bluff Check, he can spend his Bluff Token to roll again! He can't use this Bluff Token on another Skill, however.
Ruleset 4 - The Glory: When a character rolls a 20 on a Skill Check, you can improve the result of what happened removing the need of repeated tests - On your case, the Appraise Check would not only evaluate the magic sword, it would also evaluate every other item in the Hoard Pile!
You can create new things yourself, and you are not, by any means, obligated to use only one ruleset for "Critical Skill Checks". I mix Boom and Glory most of the time in my games, for example.
What you don't do, however, is to change what being successful means. If a player rolls a 20 on the Appraise check, he can get exactly what the item costs, and maybe what the item does (Appraise don't, by default, reveal the magic item's abilities), but the check should not grant the item extra abilities.
Think this way - Rolling a 20 on a diplomacy check against a random commoner won't make the commoner suddenly knows where the lost son of the Duke is hiding: the commoner don't have this info from the start, so you can't really make it appear from nothing just because someone rolled a 20.