Regarding your question of regular US English:
You can choose any forms of word spellings or phrases that you like when writing up your listings, for example color, or colour. If the Canadian flag icon next to English is showing at the top of your screen, this means that your listings will be posted in English, on the English page of ebay.ca, as opposed to French. (Open the Canadian flag icon, and francais/French should appear.) Ebay.ca will accept listings in both English and French.
There is no choice of a U.S. flag or U.S. regular English. The system will not re-spell your words according to the more common Canadian, British, U.S., Australian, etc. spellings the way flag icons showing a particular country work in Spell Checkers. In other words, this feature is not a spell checker.
Don't worry too much about your particular word spelling choices; if they may be used more in Canada or in the U.S. Remember, pictures in your listing can be understood by readers of many languages, and Clear, In-focus pictures speak most clearly to buyers.
