Normally not, but:
1. I don't know the T&C of all the sites, you'd have to check all the sites you sell through, and their partners. Stuff sneaks in, esp on partners.

2. Apparently some of these sellers have some sort of arrangement with (some of?) the agencies whereby they only pay the extended licence if they actually sell something, which makes policing impossible.

I'm actually not a stock photographer. I am just interested in selling art prints and saw someone doing this (selling them as digital downloads). I traced the images back to stock photos but I have no clue where they purchased it from (as there is multiple photos of the same photo on multiple stock photo sites). From all the extended licences I read so far, it doesn't seem you are allowed to.