What does the border color mean in Little Inferno item catalog?

Solution 1:

According to InfernoFans:

The first time you buy an item, the border turns silver. The second time, it turns dark grey. The third time it turns gold.

If you get gold borders on every item on a catalog, that catalog gets a star.

If you get a star on every catalog - i.e. you have bought every single item three times - something happens.

According to Gamefaqs, that special thing is that

when you have a golden frame around all 140 items (7 Stars), you'll get an additional letter from miss Nancy. There's no item in that letter, though.

Also, silver and black bordered items require time to restock but gold-bordered items are always in stock.