The only ways to keep your US funds in US dollars are to carry a U$ balance and use it to buy or to open an American bank account.
Carrying a U$ balance allows you to buy on many sites as well as on eBay, but you may have lot of money just sitting doing nothing for you. It does seem to be very safe there though and interest rates are not good enough to make that a huge drawback.
Opening an American bank account is NOT the same as a US dollar account in a Canadian bank.
If you bank at TD or the Royal Bank, talk to the manager about opening an account with TDNorth or RBCBank, which are licensed in the USA.
With the RBCBank, we can move our US funds to our Royal Bank US dollar account without paying exchange. I'm not sure if this is also offered by TDCanadaTrust for TDNorth.
You may see ads for these banks around "snowbird" season, but most of them are commercial accounts used by Canadian companies with a lot of business in the USA.
We have one as a legacy of our now-closed B&M business.
I opened another for my little eBay business but found the fees and other costs too high, and went back to paying exchange as being less expensive.
Note that eBay has not found a way to allow multiple bank accounts for a seller enrolled in Managed Payments. This includes American bank accounts held by Canadians
