Not all the rpg line, the Barsoom Chronicle. Is there a one or two volume set? I’m trawling internet and not having much luck 
EDIT: My Google Fu found a five book or seven book omnibus… better than nowt 
‘Princess’ to ‘Chessmen’ or ‘Princess’ to ‘Fighting Man’
I’m sorry, but I’m uncertain of your request. Are you seeking out the complete Martian series of E.R. Burroughs?
If so, I believe all his work is available on Gutenberg. But if it’s physical copies you’re looking for, I may start looking, too.
Physical copies… tbh I found books one to three in omnibuses with the next two or four… there were a few different anthologies and sets but I’m not finding any current editions in the UK
It comes down to how much you want the entire series versus the “best” books. Personally, I like the first ten (I didn’t like the last one, “John Carter of Mars,” which never felt like a “real” Barsoom book.
Some folks I talk to on various forums, however, swear that the series is best through the first three or first five books. Barnes & Noble has a nice omnibus edition of the first five books, in either cheap or fancy hardback. If you’re fine with that, the B&N choices are quite excellent.
If you search on e-bay you might find the old SFBC omnibus editions. There are 2-3 novels in each volume, and I think you have to find 5 volumes to get the whole series.
If you look for the paperbacks, I highly recommend finding the 1980 book series with the Michael Whelan covers. Those are the ones that give me the biggest feel for what Mars should look like. 
Disney released a three volume collected set in 2012 with all the novels, but it looks like it’s out of print now. I think I remember reading it might have been missing a preface too. I read them all on Australia’s Project Gutenberg.
The later books are great, and shouldn’t be skipped, especially Thuvia Maid of Mars, Chessmen of Mars, The Master Mind of Mars, Swords of Mars, and Synthetic Men of Mars.
I managed to find the first three printed by Fall River. There was also a complete 11 edition set printed way back, I found some first five or first seven editions too…