Okay I've come to the conclusion that it is completely random which accessories you get, having tested this out on my Soul Silver version. As you get more accessories it becomes increasingly harder to get them all because as you said it's like a lottery, and if it lands on an accessory that you've already got he will say "Oh, you didn't win. Sorry." So you waste 100P every time you don't win.
"In the room above the desk where you can dress up your Pokémon, you will find a room with a variety of Pokémon Dress Up pictures as well a single hiker. Speak to this hiker and each day he will offer to sell you an accessory for 100P. This can be done each day so be sure to check back for each accessory."
(Ignore the part saying "speak to this hiker each day and he will offer to sell you an accessory for 100P" as I have just tested this out and he let me buy all the accessories one after another. I'm not sure if Serebii got this information wrong or if this has something to do with me completing the game etc.)
Here are the accessories you can get:
Award Podium 
Big Tree 
Black Specs 
Blue Balloons 
Blue Barrette 
Cape 
Carpet 
Colored Parasol 
Confetti 
Cube Stage 
Flag 
Flower Stage 
Fluffy Bed 
Glass Stage 
Gold Pedestal 
Googly Specs 
Gorgeous Specs 
Green Balloons 
Green Barrette 
Heroic Headband 
Humming Note 
Lace Headdress 
Mirror Ball 
Old Umbrella 
Photo Board 
Pink Balloon 
Pink Barrette 
Professor Hat 
Red Balloons 
Red Barrette 
Retro Pipe 
Silk Veil 
Spotlight 
Standing Mike 
Surfboard 
Sweet Candy 
Top Hat 
Yellow Balloon 
Yellow Barrette 
Source: Tested out and Serebii
Hope I helped :)
