I have not used or even seen any of the three machines you are considering, so the following is based only on my perusal of the maker?s websites. Feel free to ignore my opinions.
I would definitely avoid the 4x4 machine by eBay seller Cabguy2006. From what limited info is posted, it appears to be using some type of plastic roller on an aluminum v-rails for x and z, and using the surface of the lightweight box section gantry for y. In my opinion, this will wear quickly and is not very stiff.
The Romaxx machine looks more robust. It uses steel v-rails, which are much more acceptable. It appears fairly well made. I am not familiar with this brand but it does appear that they have been around for quite a few years.
The Nebula machines are popular around here, the are several owners of it and other Probotix machines who can give more informed opinions based on their experience. The probotix machines have precision guide rails which are more precise than the v-rails and probably need less maintenance. In my eyes, I do not like the gantry design and the distance the spindle gets cantilevered out when compared to the spacing of the guide rails, and the unsupported rail for z, but again, I would rely more on opinions from those who own and use them. I have a great deal of respect for owners such as @4DThinker
and what they accomplish with their probotix machines, so my concerns may be unfounded. The pricing for the Nebula includes the computer already configured to run it, that is a definite plus, and is not included with the Romaxx.
I know that you listed these three, but what is your budget? I believe there are other machines in the $5k price range of these. Are you considering any other options, such as a kit? You may be able to get a more capable or larger machine as a kit. Remember that design software and bits will also be needed.
When you say ?glass? work are you referring to Fiberglas? Or are you looking to etch glass? I?m not sure any router can machine glass. Fiberglas eats bits and the dust is nasty, make sure to use good dust collection.