I'd avoid Gen 1 Marauders, even with the Lothar Barrel. Easy to tell because the Gen 2's have the adjustable cheek piece if it's a standard stock. Not sure what accuracy you are seeking, but my Gen 2 Marauder with Crosman barrel does quite well up to around 75 yards. Haven't been able to push past that due to range restrictions.
Watching some recent AEAC reviews, Crosman has a new barrel making process that seems to rival the Lothar Walther barrel quality. If you bought new, you'd get a new barrel from the new manufacturing process, plus the warranty. While I added a regulator to my Marauder a month or so ago, I still haven't adjusted it yet. Without the regulator, the gun performed quite well, getting about 30 shots per fill. You can always add the regulator later if needed. The Audrius Reg from Lithuania is only $69 on eBay, or buy a Huma for about $120 from AOA. I'm trying the Audrius and initial testing looks good. When the weather warms a bit (I'm a coward, and it's a blazing 9 degrees F right now), I'll spend some serious time tuning the gun for better efficiency. However, initial tests were good.
Then, there's the new factor. I like taking new things out of the box with warranties. If I buy from someone else, I don't know what care it's had. Has it been dropped? Have they done a modification to the trigger I don't know about? My paranoia kicks in on something like a gun. There's no Car Fax for a PCP!
Personally, unless there's a substantial discount for a used item, I'll buy new. Your call, probably can't go wrong either way. If all else fails, flip a coin. You really can't lose.