To sell an item you've bought, click on the redecorate button (either outside or inside your restaurant) and drag the said item into the cashier). The selling value is 30% of its original price.
To gift an item, click on it and drag it into the present icon on the far right corner of the redecorate tab. A list of your acquaintances will pop out. Just choose the person you want to gift the item to and click on the tick.
Note - You can gift items to yourself!
To make your restaurant disappear.
You may have noticed some of the restaurants being 'invisible'.
To do this,
Go to Street, then click on redecorate.
Buy a new roof and base, then send the old one to someone (the old roof and base will appear as boxes in your inventory). Do NOT click the tick and refresh the game.
When you reload, you SHOULD have a totally invisible building.
However, bear in mind that if there is absolutely nothing to click on (not even doors and windows), your friends won't be able to enter your restaurant.