I am selling my used tires to a Canadian Friend. Do I declare this intent when crossing the bridge and do I pay HST tax at the Bridge?
Yes, as an American visitor you should declare them at the border crossing. Best to bring an e-mail or whatever where you agreed on the price. Tell them it's a friend (ie. not an arms-length transaction) and mention that they are used when you talk to the guy or gal in the booth.
If they ask you to pay any tax, it will be paid at the border (they'll ask you to go inside with a little slip of paper). I expect at most it will be the HST, though technically it could be more if the tires were not made in the USA, Canada or Mexico etc.
Chances are pretty good that they will just wave you through and wish you a good day.