Infiniti - G35 :: Rear Main Seal Leaking After Engine Replaced

Last year, I had the unfortunate experience of having to get my engine replaced on a 2003 Infiniti G35 (only 121k miles, I got it used in '06, long story, I assume previous owner didn't take very good care of it). I opted to have a local mechanic find a used engine instead of paying for a rebuilt one and he was able to find one that had ~60k miles on it so I went with it. Now it's 9 months later and I have started noticing a burning oil smell while sitting in the car at a stop. I took it into a shop (not the one that replaced the engine, different tire and lube shop) and they found that the rear main seal was leaking pretty badly and that's where the smell was coming from. They don't do that kind of work so I need to take it somewhere else to get any repair done.

I feel like, if a second diagnosis also says RMS is the problem, that the shop that put in the engine should have given me a new RMS when they put in the used engine. Is it not something that would be visible and thus easily replaced when installing the new engine? If they did in fact replace it, it wouldn't possibly have failed on its own after only 8 or 9k miles, would it? I don't normally hearing about RMS problems until cars are well over 100k miles.