Cleaner Still:

partNum = InputBox("Please enter the part number", "Part Number",
0)

Sheets("Sales").Range("B1").Value = PartNum

While Sheets("Sales").Range("B1").Text = "#N/A"

Output = MsgBox("You have entered an invalid Part Number",
vbRetryCancel, "Invalid Number")

If Val(Output) = vbCancel Then Exit Sub

Loop