Best Vendor Plat Mobs for level 15-20?

[Defo]

There's also the entrance guard in Highkeep, can't remember his name - but he's level 20 and is on a 2m respawn. Drops a fine steel sword every time.

There is a pathing guard you have to watch out for -- but with a Slow, the enchanter pet could probably kill the 2m guard easily.

[Belambic]

Greater Lightstones from the wisps, as has been mentioned, will net you a lot of experience and an item worth 9 plat on selling back.
Bandit sashes which are easier to get sometimes, will get you experience and a bronze weapon which sells for several plat each.