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:
Source: Tested out and Serebii
Hope I helped :)