Perhaps you can use Data Validation for this... no programming
Select the list in Sheet2 and name it something like List through Insert|Name|Define
Then go to the input cell in Sheet1 and go to Data|Validation
Select Custom from the Allow options and enter formula:
=ISNA(MATCH(A1,List,0))
Where A1 is the input cell
Then select the Error Alert Tab and enter a Title and message you want to appear.
Click Ok. Test it.
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