Some agencies will send your art to dozens of questionable "partner" sites.  Different pseudonyms help determine which agency is sending your art where. 

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Any advantage of using different pseudonyms for different agencies?

Personally, I think it's good your own personal brand that your real name appears which can be used later to show potential clients.

And personally I'd agree. Name and brand or whatever some call it, is better than every site, different, no identity. Plus it will look like you are stealing from others because all the accounts are different. I see no advantage. Doesn't stop theft or protect anything.

At Bickstock I get 30 Cent per image, at Alamy 10 Dollar, for one and the same image. If any Alamy buyer finds out, that my images are 30 Cents, he or she will always buy from Bigstock.
This is my idea...

With reverse image search, any idea that you can hide where you sell from a buyer (if they want to check this out, and many don't) is outdated, IMO.

I left Bigstock after they introduced their cheapo subscriptions and CanStock after they became primarily cheap subscriptions, but I do sell at sites with subscriptions  and also at Alamy, with my real name used everywhere. Alamy offers different license terms, and a very extended payment schedule. As we're licensing images, it's fine for prices to be different for different terms - it's not the image we're selling.

Buyers should pay more if they want to use the image first and pay months later, versus the model at most of the micros where you pay up front. There are also differences in the amount and quality of support a buyer gets, all of which can influence prices.

Years ago, when I had a free image of the week (I think at CanStock), I sold that image that week on other agencies. Some buyers shop around, but clearly many don't; they find a couple of agencies to work with and stick with them until the agency lets them down in some way. Then they move on to another couple of agencies.

YMMV

At Bickstock I get 30 Cent per image, at Alamy 10 Dollar, for one and the same image. If any Alamy buyer finds out, that my images are 30 Cents, he or she will always buy from Bigstock.
This is my idea...

I've sold two images for 100$ net at Alamy last year and both are avaliable cheaper at other agencies. So, yes, some will go and buy from cheaper sites and some will not.