Calculate number of days between sales and prior sales date

In column A is a list of car sales
In column B is the date of the sale

I tried using a vlookup but that grabs the first date in the list and I am trying to grab the second to last date in the list. I have attached a sample spreadsheet with desired output. Thanks any advance for any comments.

An example would be

Column A ...........Column B
Ford...................3/1/13
Buick..................3/8/13
Ford...................3/9/13
Ford...................4/4/13
Buick..................6/6/13
Ford...................7/1/13

I'm trying to figure out the days elasped between sales for same make of car. Desired output would be:

Column A ...........Column B..............Prior Sales Date (col C)............days between sales (col D)
Ford...................3/1/13.................n/a; first sales date.................n/a; first sales date
Buick..................3/8/13.................n/a; first sales date.................n/a; first sales date
Ford...................3/9/13.................3/1/13....................................8 days
Ford...................4/4/13.................3/9/13....................................26 days
Buick..................6/6/13.................3/8/13....................................90 days
Ford...................7/1/13.................4/4/13....................................88 days

C2: =IF(COUNTIF(A$2:A2,A2)=1,"First instance",LOOKUP(2,1/(A$1:A1=A2),B$1:B1))

copy down as far as needed