There are several things to know that might make this easier.
First, Quicksilver uses the system’s Find Pasteboard, so whatever you search for in other (correctly written) applications will automatically show up as the search term in Quicksilver.
This also means that when you select text, you can hit ⌘C to put it on the regular pasteboard, but you can also hit ⌘E to put it on the Find Pasteboard. If you do that, the term will automatically be there the next time you use the Search For… action.
There are tons of options for when you already have something in the regular pasteboard, though.
The most obvious is just to hit ⌘V in the 3rd pane when using Search For…. If you have the Spacebar behavior preference set to “Smart” (recommended) or “Jump to Argument Field”, you can just select a search engine in the first pane, then hit Space to jump to the third. To search for the current clipboard contents using DuckDuckGo, I just have to type
G Space ⌘V ↩
You should also be aware that the Search For… action is reversible. Instead of
Search Engine ⇥ Search For… ⇥ term you can do
term ⇥ Find With… ⇥ Search Engine. So, you could call up Quicksilver, hit ⌘V to put the text in the first pane, then select the Find With… action. The search engines are sorted by how often you use them, so your favorite should be the default.
Finally, if you do this regularly, you could just create a trigger for
Clipboard Contents ⇥ Find With… ⇥ Search Engine. (This will be a little tricky since the Find With… action won’t appear for the Clipboard Contents item. Just enter some text in the 1st pane, select Find With… in the 2nd pane and copy it with ⌘C. Then, go create the trigger by putting Clipboard Contents in the 1st pane, go to the 2nd pane and paste the action with ⌘V, then select a search engine in the third pane.)