I can add some info on Romeo y Julieta ["RyJ"]. The company is known for a pair of mid-1920s boxing issues as well as some nonsports movie cards. The front format of the cards on eBay roughly matches with a rare boxing issue:
Here is the principal boxing issue:
The cards are real photo, so there should be signs of silvering and very difficult to duplicate photographic age on them. Backs are found with black or [very rare] brown printing. There are tone variations among the cards with some sepia and some grayer. Also, there is a pretty good variance in size.
I have no idea who the seller is, so no opinion there. Is there another thread on these?
In terms of potential for discoveries, I do a lot of research on Latin American boxing cards and new issues surface seemingly every year. Last year two Cuban tobacco boxing issues popped up. New cards also surface periodically from established sets, most of which cannot be definitively checklisted.
There are two or three guys here I can think of on this board who have even more experience than me or anyone else in the world handling RyJ cards. Ryan Christoff is one of them. The others may not be 'out' so I won't mention names, but certainly any of them could examine these cards and give a pretty solid opinion on whether they are consistent with the genuine RyJ issues.