The mask shop need help

Im not so sure about what do you mean but ill answer as i see it.

In the start when the Happy Mask shop opens, there should be the keaton mask, which you must find owner, after you've done that you can start looking for another fren who needs a mask.

And finally, when you find the bunny hood owner, you can get any mask you want plus some extra masks.

i think u hav to go farther in the game first.
beat a dungeon first and try again.

Check www.gamefaqs.com they'll know what to do

Here is what you need to do :

1.) Get the Egg from Malon outside Hyrule Castle.

2.) After one day, the Egg will hatch into a chicken.

3.) Wake up Talon just outside the side of the castle.

4.) Go through the castle and get the Letter from Zelda.

5.) Show the Letter to the guard outside the Kakariko Village Death Mountain entrance.

6.) Get the Keaton Mask from the Mask Shop in the Market and talk it to the Kakariko Village Death Mountain entrance guard while wearing it to sell it.

7.) After you give the Keaton Mask, go back to the Mask Shop and get the Skull Mask.

8.) Go to the Lost Woods and take your first left. Stand on the short stump with the Skull Mask on and talk to the weird guy dancing on the taller stump in front of you.

9.) Go to the Mask Shop, and get the Spooky Mask. Go to the Kakariko Village Graveyard during the day and talk to the kid while wearing the mask.

10.) Go to the Mask Shop and get the Bunny Hood. Find the man running around Lon Lon Ranch (appears after beating Jabu-Jabu) and chase him until he stops. When you leave the Market, try going to the right side of Lon Lon and you may see him running in the distance. Don't worry if you don't see him right away, after a few laps he'll be there. While wearing the mask, talk to him and he'll buy it.

11.) Now you'll have access to the Mask of Truth, the Goron Mask, the Gerudo Mask, and the Zora Mask. The Mask of Truth lets you talk to the time-keeping statues, and the other masks are just for fun.

Are you sure you did get the Egg from Malon ? ???
If so did you wait a day and take the hatched egg to Talon ? ???
Did you then get Zelda's Letter ? ???
Did you show the letter to the guard near Death Mountain Entrance ? ???

Only if you did all of that then you can get a mask.
If this doesn't work you must haved used a cheat or something.

I remember having an odd problem like that on my n64 have you tried reading the rules to the left of the shop owner?

it's really annoying but there is a specific way to get the keaton mask,

also are you using any cheats? since they could interfere with it.