Dubi Zwiebelmuster is the same company I posted above !! I have no idea what Zwiebelmuster means by its use in this mark, Dubi is a town name I think !! The marks can be different if some pieces were purchased at a later date than the original set !! Many mfrs changed marks over the years !! It is not unusual to purchase a set and add additional pieces as the need arises !! People do that today !!

https://www.ebay.com/gds/Meissen-vs-Meissen-Blue-Onion-Porcelain/10000000006290576/g.html

Your set is a copy of Meissen Blue Onion pattern !! But don`t think it is not good because its a copy !! It is still a very well collected set from a good company !!

Here’s another try at uploading the pic.

The pic either did not load or there is a problem with it !! Can`t see it !!

I took a pic of the blue onion plate. The stamp has more on it. Thoughts? Any idea how much to sell for?
Thanx for your help.

According to Porcelainmarksandmore the mark on the Royal Princess china is likely a decorators mark,,not a factory mark !! Even though the china was made in Germany many factories sold much as undecorated white ware !! No telling where this was done,, could have been decorated anywhere !!
The best way to sell as a set would be your local Craigslist or any local site !! Just be careful and do not let anyone in your house and always have someone with you when they pick it up !! Weird people in this world you know !! And don`t fall for the scammers !! There are a lot of them prowling the sites !! If you are not familiar with all they do just ask here !! I am usually here daily !!
China has been in a down market lately so check around for pricing !! Not asking prices just selling prices !!