Selling and Buying primary house, possible to close at same time?

[Robert Hastings]

i am about 4 months away from our listing our house.  over the next couple of months, after the holiday, we will spend time and money into our primary to position ourselves as best as we can be with regards to the highest sale price.  new paint, upgrade bathroom vanities, upgrade laundry room, fix this, fix that, etc...

the sale price is going to govern our budget for our next house.  i feel we can sell our house 270k and i figure we can net 260k from the sale price?! ( our mother in law is the real estate agent and she will waive any commission on her end...what should we expect to net off a sale price of 270k in new jersey??) if we can net 260k, we should have a clean 100k left over after we payoff our mortgage.  that 100k will be our down payment for our next home which will place our next home's budget within the 350-400k sale price...

the X factor is my wife is pregnant with our second child.  she is one month pregnant.  i know there are alot of variables with selling and buying real estate.  i am trying to plan and budget and coordinate and navigate all options that will allow us to sell our home, buy another home before my next child is home from the hospital.  

how does this work?  how should i approach this?  should i get a pre-approval letter from my loan rep, make an offer on a house contingent on selling my primary?  should i be as upfront as possible with both buyers of my primary home and sellers of my next home begging and pleading to close as soon as possible to each other so i can avoid being homeless and renting with a preggo wife lol?  

what governs the close date with real estate transactions...lenders, correct? how can i evaluate the strength of my buyers offer besides the amount they want to pay...meaning can i ask to speak to their lender to see how tight their credit/DTI/bank statements are....last thing i would want is my buyer's loan falling thru two weeks before i close. the domino effect would prevent my sale, which would prevent my buy bla bla bla i need a drink!

i really dont want to sell, then rent for 6 months, then buy but that is always an option....last resort option but its there.

thanks in advance