you need to have enough simoleons to purchase the property click on the venue and you should see an option purchase property once you click on that your sim will go to city hall to buy the property
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p.s
you really don't have to own the bakery as your sims can bake at home but wait for the sim to put the plate down then drag and drop to the fridge and your sims can eat the goodies as left overs or put the goodies in your sim inventory then go to the bakery you can use the camera to zoom to the bakery then drag and drop the goodies from your inventory to the display case so if you want to full the display case up while at home but not own the lot you'll need to bake at home drag and drop to the inventory then pan the camera to the venue and drag and drop to the display case if you do own the venue then bake at the venue once the sim puts the plate down then go to buy mode then grab the plate and place in the display case back to live mode bake another plate back to buy mode grab the plate and place in the display case then back to live mode
