Sell and Buy houses from bank in the same round?
From the current rules at Hasbro, under the Selling Property section (emphasis mine):


  Houses and hotels may be sold back to the Bank at any time for one-half the price paid for them.


And then from the Houses section (emphasis mine again):


  When you own all the properties in a color-group you may buy houses from the Bank and erect them on those properties.  If you buy one house, you may put it on any one of those
  properties. The next house you buy must be erected on one of the unimproved properties of this or any other complete color-group you may own.
  
  The price you must pay the Bank for each house is shown on your Title Deed card for the property on which you erect the house. The owner still collects double rent from an opponent who lands on the unimproved properties of his/her complete color-group.
  
  Following the above rules, you may buy and erect at any time as
  many houses as your judgement and financial standing will allow.


So I see no reason why you couldn't sell a house and buy it back again in the same round.  Just remember that you have to sell houses evenly as well.