The vast majority of it can be thrown away and is not necessary to regain old items. A few exceptions:

You need to keep the slippers if you want to trade them them back for the swamp boots.

Magic gauntlets were were exchanged with an engagement ring. You can the ask the person who has them to get them back, but I can't remember if the ring is required as well, and the site I am looking at is not specific.

The urn is not needed to regain the exchanged item (an ordinary crossbow) but you can give it to someone who will pay you for it.

You can choose to trade the breastplate for the lost magic armor but it is also possible to simply take the armor back.