Warranty should transfer...
Unfortunately, it doesn't. S&W's warranty is to the original owner only. They don't always ask, and if the original owner didn't "register" (i.e., mail in the warranty card, or do it online) the gun with S&W when they purchased it (not required) S&W probably won't have it on record as purchased. If they ask for proof of purchase, though, the warranty claim would be denied. Besides, from a personal ethics standpoint, it isn't fair to S&W to try to "slip one past them." (I'm not saying anyone here would, just a general comment.)
It's very hard to find new SDVE pistols for sale, especially in Academy Sports...when they had them on sale for $289, my local store got in 20 of the 9mm models, and sold them all within two days. None of the stores in the DFW area have the 9mm or the 40 models. I bought a SD40VE in a local shop for $339, which was higher than I wanted to pay, based on Academy's regular price of $319, but the $20 difference was what I'd have spent in gas driving all over trying to find one.
Does the SD9VE come with two magazines? New, they do...and if this one doesn't, that's another $40 right off the bat you'd need to spend.
It's hard to know why someone is selling a barely used gun. Maybe they need money...maybe it was an impulse buy, and they just don't like the trigger...maybe they bought it for their spouse and the spouse can't handle it. Knowing the real reason for the sale is dependent on the honesty of the seller, and that's not always a known factor. If the warranty transferred, then you'd be covered...but since it doesn't, you'd be on the hook yourself for any issues.
Unless money is a very big issue, I'd pass on a used gun that is still in production, unless the price was a LOT lower. The M&P sounds like a better deal price-wise, since it's about $200 less than new retail. LEO trade-ins are usually not fired that much, but they can be rough cosmetically...it doesn't affect the function, but still it's a factor.
All things considered, I'd buy a new gun if I possibly could. Some guns are no longer in production, so if you want one (like a S&W 3913, for example) you have to buy used...but other than something like that, I'd buy new.