excel - Complicated lookup - based on specific value in list -

i need in finding formula can specific lookups.

i have accounting spreadsheet 30,000 rows - example extract below:

product nr. product date    purchase or sale    sale tat 12345   test product 12345  02.06.2014  purchase     12345   test product 12345  02.06.2014  sale    0 12345   test product 12345  30.09.2014  purchase     12345   test product 12345  30.09.2014  sale    0 23456   test product 23456  08.01.2014  purchase     23456   test product 23456  20.01.2014  sale    12 23456   test product 23456  12.06.2014  purchase     23456   test product 23456  13.06.2014  sale    1 23456   test product 23456  30.07.2014  purchase     23456   test product 23456  04.08.2014  purchase     23456   test product 23456  04.08.2014  sale    0 56789   test product 56789  07.01.2014  sale     56789   test product 56789  13.05.2014  sale    126 56789   test product 56789  03.12.2014  sale    204 

each row either purchase or sale. need solution can, each sale, lookup previous purchase date - , calculate difference.

right now, lookup previous row , compare dates - of course, totally inaccuate - 1 purchase may have many sales. check if product nr same, that's all.

any ideas on how can say: "for sale... go last purchase in list... , use date."

thanks!

so, @kyle 's answer solution:

index(c$2:c$33, if(d30="sale",max(if(d$2:d$33="purchase",if(c$2:c$33<=c30,if(b$2:b$33=b30,row(c$‌​2:c$33)-1)))),""))  

thanks!