Here is the best way I found to do this without disrupting the existing household. If, like me, the ancient library is in the Madbouli house and they are both merchants that never come home. (I found the "ring doorbell" option never came up. I wouldn't have any interactions to access the house at all. So some of these responses didn't help me.) The best way I found to handle this is to use the TestingCheatsEnabled True cheat. Go to the market where the merchants are, that own the house. In my game, Fahad Madbouli was the Food Merchant and Sanaa Madbouli was the Book Merchant. Since my sim befriended Fahad, I will use him as an example starting now. Hold Shift and Click the Food Register. Click "Object...." and then "Delete It". Wait for Fahad to start walking home. To restore the Food Register, hold shift and click on the ground in the market and select "Build on this Lot". (You would think it would be "Buy on this Lot" but the item you need is in Build Mode). Once in Build Mode go to the Community Objects Icon (It looks like a store). Click on the Misc Objects Tab and you will find all the registers in there. Once you continue game play, Fahad is on his way home and greets you and lets you in his house and a new merchant has replaced him at the market.
P.S. I'm sorry I'm years late on this....I've never found a satisfying answer until now, so I hope someone out there can still benefit from this... now that I'm sure most people have moved on to Sims 4.. but I love the Create-a-Style tool too much to move on to Sims 4, myself.
