The question is a little very dicey but lets’s take it one step at a time. If you used a broad keyword (+keyword +selling +point) and the goal is to bid for all keywords relating to ‘selling point’ then i recommend you should not add ‘selling’ as a negative keyword because you might confuse the Adwords system and any query with ‘selling’ might not show on the Search result .
Recommendation: I recommend you figure out another way to pay attention to your Search Terms report & exact match any irrelevant searches that appear. [Selling Point Parkway], [Gross Selling Point] — Search term report is a great tool, sometimes you don’t know what you’re showing for until you look there & then you add your Negative Keywords to avoid waste of Adwords Campaign budget. Certainly, I think the question ‘On AdWords, how do you add “selling” to negative keywords when bidding on “selling point”?’ as been answered.