You need to install ambitions and world adventures but if i were you you should install ambitions. If you want to make your own store you need ambitions(and world adventures i think) for the consignment cash register and if you cant find that there are loads of forums telling you where you can find it. Again you need the sims 3 ambitions.
Chat with our AI personalities
No, unfortunatly you cannot, and if you were to marry or otherwise bring the specialist into your household, he/she would use his/her job and be replaced by another npc.
you click the regester and select buy things. then there will be a tab saying sell things or soemthhing along those lines
The grocery store.
From the Sims 3 Store. You have to buy it (which is annoying, since we already pay for the game...).
there is no link you have to but the game in store then install it