I think you are both talking about selling from different inventories.
Selling from the HOUSEHOLD inventory creates a popup, if you have more than one item in a stack, and asks you how many you want to sell.
Selling from the PERSONAL inventory allows you to drag a single item or an entire stack to the sell panel but you can't specify a specific number.

What happens to children/toddlers/babies when their parents/all adults in the household die?

About the flea market - Is there ever anything else on sale except chairs, couches and lamps?

I really like the idea of the flea market, but it takes so long until I actually get one that I usually have earned enough money by then to buy things via Buy Mode. And every time I went there I just saw these three items there (and of course the minerals and dolls on the tables).

OK, stupid question time - how long does it take (sim time) for a sim with maxed out skill + inspired mood to paint a painting without mods? I've tried to find information about that and writing, but, uh, no luck.

Nope. The rugs only ever sell those three tiems. I think the idea is that there is a "used" set of furniture that can only be obtained by purchasing from the Flea Market (without cheats) that is made up of those objects. I think it's random if those specific items actually show up though.

Is it possible to sustain a relationship for sims with a "childhood tormentor" past?

I guess this is what appears at the relationship panel when sims grow up as enemies(two are brothers and a girl was also raised in the same house but isn't related). They were enemies as kids and teens but eventually I decided to make them work it out, be friends, etc. but it still shows up in the panel. I do think it should show up(I think it's a nice feature, like a memory in a way) but then even being friends they get a moodlet of "hating family" and usually it's enough to override the other moodlets. In an attempt of making things up between the girl and one of the brothers, I managed to get then romantically involved(steamy shower, watching romance on tv, etc.) but anytime these expire the two hate each other again. So I have to be constantly watching out otherwise they will blow it all away, especially because she's hot-headed.

Guess it was probably stupid to get the two engaged, so back to the original question, is it possible? Or am I setting them for divorce(or being left at the altar)? I think the brothers can remain friends as soon as they move out and take it easy from afar, avoiding being together all the time so the old grudges don't surface. But the other two as a couple...if I manage to raise the relationship to high or max, do they stop hating each other? Has anyone played this situation?

Is it just me, or can Sims only use the Rude Introduction on stray animals?

If you have MCCC installed, they are moved into other households. Without the mod, they get deleted.

babies get deleted when left alone but not toddlers or kids with MCCC installed.

Is there any way to build a custom apartment with CL? Any mods or something I can download to make my own apartments?

@joof CityLiving provides us the apartment shell which cannot be customized. You can only build inside the existing apartments or in the case of penthouse, you can treat them like normal house, but on top of a shell.

How do I bring Doc the cat back to life with the Ambrosia pet treat? I moved Doc the Cat ( he's a ghost) in with my sim I bought the Ambrosia treat. but, it only treat it will show is for to give him big pet treat. I tried putting in the floor in front of him for him to eat it, didn't work he just fell asleep by it. I tried putting in a pet bowel for him to eat and it would not let me. I tried to get a friend of my sim to visit and give it to him, that did not work either. Could it be because she owns her own vet business. is that why?

Strange. Did you adopt Doc? He's part of the Lynx household originally. If you didn't adopt him, maybe that's why. Otherwise it should work like when you give a Wellness or Aging treat (i.e. you just give the cat or dog the treat).

I just moved him in from manage households in main menu, I found him by unhide hidden households with mcc. Maybe that's why? Maybe I need to find his urn, pet gravestone and bring it on lot with him

I bet that's it. I don't play with ghosts very often and definitely not ghost pets, since I have aging off, so I wasn't sure if you could/had to adopt Doc or not. However, now I remember ghosts are moved into a hidden household since the culling changes, so it might have bugged him out somehow unhiding his household since that can't be done without mods. I'd try to find his urn if you can and then see what happens.

I just started playing the game and I couldn't find this anywhere- how are traits assigned in children? Are they inherited?

Nope. As long as you don't have mod for this, you choose traits and aspirations just as you like.

I think they do inherit some 'hidden' traits - I had a child who got negative moodlets when there was something broken or dirty, but I hadn't given her the neatness trait. Her mother had it, however. She lost it when she aged up and all her traits were chosen for her. Maybe it was just coincidence - the game just randomly filled that hidden trait slot?

Is there a way to create a pet rather than just adopt one? I want my family to own a specific breed with a certain personality.

Yes, just go into CAS, add sim and make it a pet. You might need to enter full mode though.

I hate to ask another question, but I built a house and I'm trying to access the cinematic/cameraman mode from build mode to get screenshots of it. Is that possible?

Sadly, I don't think so. You have to sell everything one by one. Someone should really make a mod for that though.

tab button on your keyboard Don't know why you want to do it in build mode?

Actually a found a cool mod here: http://modthesims.info/download.php?t=595418

tab only works for me in live mode. I don't want to put anyone in the home, I just want to take pictures so I can't use live mode.