As an Indie, I don't work with an agent currently, mostly because there's no need. There's no publisher, studio, or third-party corporation who is trying to buy the rights to my work or to whom I am trying to sell my rights--be it in the US markets or International.
The moment a business of any ilk wants to buy any segment of my rights, that's when I'd look for an agent, or at the very least, an IP lawyer. A business is always out for its best interests, which usually aren't my best interests. I suffer no delusion of being able to outsmart an entire legal department for whom IP contracts are old hat, thus getting an agent who is well experienced (and/or whose agency is) would behoove me.
Would an agent take me on? Possibly, if I already have an offer from a large publisher. Depends on whether there is money to be made--now and in the future--that is worth their time. Would an agent take me on with just the catalog I have and no offers on the table? Oh, gosh no. I have nothing for them to sell.
What if you're that one-in-a-billion unicorn Indie author who is approached by a publisher that wants to buy your already published works to which you still own all rights? Congrats! YES, get an agent before you sign anything. Reputable publishers will not balk at you asking them to hold that thought for two weeks while you secure an agent. When querying agents at that point, be sure to put "Query: Have Offer From [Publisher Name]" in the subject line.