Hope I get you right.

Stock on sheet "Stock":

prod stock
abc 13
abb 44
edd 33
gdd 66
uhh 16

Sales on sheet "Sales":

prod sales Stock
aaa 15 0
abb 22 44
err 19 0
dddf 30 0

Formula in column C for finding stocks:
=IF(ISNA(VLOOKUP(A2;Stock!$A$2:$B$6;2;FALSE));0;VL OOKUP(A2;Stock!$A$2:$B$6;2;FALSE))

You may have to replace the semicolons with commas.

Hans