Buy it... it's just seeping, the cover isn't under pressure so it shouldn't leak bad enough that it it drips on the ground. I, personally, resealed my timing cover myself because I can't stand a leak of any kind. Took me about 20 hours, replaced valve cover gaskets, intake gasket, water pump, OEM serp belt for the Tacoma 1grfe (half the cost, same belt size, just sourced differently from the 4runner) and used the Toyota FIPG gasket maker to seal the cover.
Just keep an eye on the oil level (Hint: pull the dipstick out and let it sit for 10 to 15 min, then check the oil level, that stick can be hard to read sometimes... you'll see what i'm talking about when you check it after its been running).
The timing cover leak shouldn't be the reason to pass up an otherwise perfectly good 4runner.