Most collectibles can't be sold back to the shop, but add-ons can be sold back I believe. It will not give back the amount you paid to buy them though.
The Buyback Price listed on the collectible/add-on info thing will tell you how much they can be sold back for.
You can sell back some mailboxes for a reduced price, but that's about it. This is for if you already bought one and want to "upgrade" to a better one.
And of course the turnips right now... but that doesn't really count.