My friend just brought me a gift last weekend of most of RM2 books, including the core box set with the box in fantastic condition. All of the books look unused. He said he ordered it from an auction site (not eBay) and it was shipped from UK to the US. He said the price was great but being a gift, he didn't say. He saw the condition of the books I use and he thought it would be a great gift for me to keep as a "shelf copy" to go with the "working copies. RMC-I actually has all of the pages still bound! Seriously, the books look like they were never opened.
The books exist, and they can be found mostly in US and UK. I've not found any great deals in Canada which is strange. For Christmas 2018, I bought all of my regular players copies of RMC-II and Elemental Companion as gifts, four copies each, all in excellent condition. It takes time, but you'll find them.
As for RMC-VI and VII, I have to agree with Corey Magel. I think ICE just didn't print as many. Of course I have no proof, but RMSS was coming out and focus seemed to have shifted to the new system. Also, RMC-V and VI were "ok", probably not super sellers and perhaps ICE cut back on printing based on sales of those previous books.
For me, Finding a good copy of Sea Law was a challenge. I have only one copy of that tome.