From the photos I'm not convinced that those are reloads. I've reloaded a lot of rounds, and quite a few .303 British, and that brass has the look of unfired to me.
U.S. brass in particular in .303 caliber typically has a slightly undersized base, and military chambers are oversized. When fired there is a bit of a bulge just above the case head that is not normally removed when resized. I can't see any sign of a bulge in your photo. If there is a bulge, that is a dead give-away to reloads.
I was suspicious when I saw the FMJ bullet, as it appeared to have a slightly different shape than typical .303 FMJ, but then I looked at these factory Remington 174gr FMJ rounds and they look identical to what you have:
http://media.midwayusa.com/productimages/880x660/primary/781/781534.jpg
The dents don't bother me. They don't look like lube dents to me, and I've seen a lot of lube dents. They look more like handling dents, or likely someone cycled them through a rifle to test for function without firing. Even the one round that doesn't match could have been from someone opening the box, losing a round, and just replacing it with another round.
Two questions: 1) Was the box they came in the same as the one in the Midwayusa photo above, Remington 174gr FMJ? If so, they look like factory rounds to me.
2) Is the bullet in the one mismatching round exactly the same as the others? It's unlikely that a Frontier bullet would have the exact same shape as a Remington bullet.
I don't blame you for not wanting to shoot someone else's reloads, especially at that price (or any price). If there's any doubt in your mind, definitely take them back to where you bought them and see what they say, but from your photos they look factory to me.
Mixed in with them, however, was a few (dozen+) .44 Magnum rounds with Keith-type lead bullets. He pulled the bullets to salvage the cases...and found them to be COMPLETELY FULL, RIGHT TO THE BASE OF THE BULLET!
Off topic to .303 ammo, but it's quite normal for magnum loads to be full to the bullet base, especially with something like H110 or 2400. I totally agree about not firing unknown reloads, though. I've made enough dumb mistakes on my own when reloading. You just never know what someone else did.
Another question, out of curiosity: is $50 a typical price in your part of the world? I know civilian ammunition is often expensive "on the other side of the pond", but that seems pretty high. Is .303 Brit hard to find as a caliber?