Recent 2006 Sport Owner
I can tell you from my research that most dealers are about 1-4k over book value on used 4 runners. I looked for about 2 months within a 250 mile radius using Cars.com, autotrader and then scouring all local toyota dealer sites. Used sport models are fairly hard to find (2006 to current) but the ones that were on the lot were very overpriced and since there are people out there looking for them the dealers are jacking them up.
I was psyched that I found a 06 4wd sport with the V8 and 38k miles for 18,999. The vehicle is in mint shape and was a single owner not a lease. I basically saved about 40% off a new one and you can hardly tell.
Assuming you can look over the vehicle, drive it and assess the condition, I would strongly recommend looking at non-Toyota dealers as I got mine from a small Chevrolet dealer that took it in trade for a guy that wanted a larger Tahoe. Most non-Toyota dealers are probably going to give you a better deal, IMO.
Mine is not certified, but you can spend $20 and do the carfax yourself and even buy another vendors warranty for around $1500 if you choose (I don't think it is neccessary). Of course if you are trading in they will low ball you there, but I negotiated for about 3 hours to get my deal and part of my rationale was that the whole time I was there not ONE person came in..
Good luck on your search.
P.S. If you can get the V8 don't pass it up... well worth the couple MPG's you lose