"I sell the umbrella."
I thought L' was only used for feminine words. Is this an exception to that or did I mix up the rules somehow?
L' is used as article in front of words in singular beginning with a wovel . . . nomater if feminine or masculine, e.g. l'orso, l'anatra
You might be thinking of the rule for the singular which applies to nouns beginning with a vowel:
"Un ombrello" (masculine), but "Un'anatra" (feminine)