What kit lens do you have for the 7D that mighgt sell used for $500? The 28-135mm IS might go for $200-$225, and thats the only kit lens I can think of.
I'd recommend the 17-55mm ef-s as a first lens. If you like only standard to mild telephoto, the 24-105mm L is within your budget. The 15-85mm EF-S is also good for the price. Is that what you have??
+1. But...I'm guessing not the 28-135mm since that's a USM lens with FTM. The 7D also sells with the EF-S 18-135mm IS, also not worth anywhere near $500 on the used market. Some retailers bundle other lenses (I think there was an EF-S 18-200 kit at one point), but even that would likely not go for $500.
The 15-85mm has also been bundled, may sell for over $500, but has FTM which doesn't match the OP's statement.
I second the recommendation of the 17-55mm, with enough for landscapes, fast f/2.8. The 15-85 would also be good, f/5.6 at the long end isn't great for fast-moving dogs but would do fine in decent light.