Possible reasons:

They don't want relatives/friends to know what they have bought.

The item is valuable and they don't want the local pond scum to know they have it.

They don't feel the need to display their trophies like peacocks tail feathers.

They are on selling items and they don't want buyers who know they have the 2 different accounts knowing how much they paid for the items.

They quite rightly think it is nothing to do with anyone else what they have bought.

I am sure there are many more but I see nothing suspect about a buyer making their feedback private as it must be at 100% and doesn't tell you anything about the buyer's trading practices.