I also just changed the oil and am considering changing the differential fluid.
I've already put multiple hours into listing, uploading pictures, meeting the guy to test drive, responding to texts, phone calls, etc.
Because of the relative value of the car, I intend to meet the buyer at the BUYER'S bank to do the paperwork and see the cashier's check drawn in person (there are lots of cashier's check scams these days).
Assuming I sell it to the interested buyer, it's a fairly conservative estimate to say it'll take me 6-8 hours with the one buyer. If I list it more widely on, say, craigslist, I get to deal with finding time to meet a bunch of tire kickers, THEN finding the time off work or on a saturday morning to go to the bank to do the transaction.
In California, if the vehicle is 5-6 years old or more, the seller is also responsible for getting the car smogged; likely if you took it to a CarMax type place they'd take care of all of that stuff for you (just a guess though).
In the end, I am LIKELY to make, say, $5k over what a dealer would offer me, but there's no way of being sure that I'll get 1 cent over what a dealer would offer.
Ultimately it depends on the market. I only buy fairly unusual cars; my previous car took me 4-6 months to find, the car I'm selling took 4 months to order. I once took 6 months to sell a purple M3.
Done right, enthusiast cars hold their value well, but the target demographic is smaller so they take longer to sell.