Think of the many orphaned exotic for a while wildcat cartridges many of which you cant even get reloading components for. What a crying **** waste. There already exists a cartridge to do anything anybody wants to do. It ****es me off to see mfgrs continually coming out with new stuff that copies existing stuff just for the sake of being new. However it is true that a concentration of interest like here on this forum exists that loves to discuss all the new experimental futuristic stuff etc. You always have to ask yourself where will that cartridge (that was a hot flash in the pan when introduced) be in five or ten years. I have a pretty much golden rule. If the major retailers do not stock the ammo then avoid in like a bad disease.
I own a 7mm Rem Mag as it is the most powerful round with the best trajectory of any cartridge in the top TEN list of cartridges sold around the world. Indeed likely the top five and that is saying something because the top three exist primarily because of the Military.
I was in a pawn shop not long ago and there was a super exotic Weatherby caliber that the guy could not sell for $150 with scope because it was a cannon intended for Africa and no deer hunter would ever want it to knock his shoulder off..... anyway... getting off the subject. Get something that is universally popular and that AINT a 280AI. Honestly I have seven rifles and 14 pistols and I load for five calibers and I dont even know what a 280AI is without googling. Unless the auction attendees are all members of a reloading club, it would be stupid not to pick the 7mm Rem Mag. This assumes no other options are available. If there was, I would auction a 270. The 270 is more efficient and equally as popular as the 7mm Mag.
On the other hand, If I were you I would scour all the local pawn shops real quick to see what is available cheap that could be cleaned up for presentation. You might find a desperate seller with a very nice condition gun. when you are holding it up in front of a gun crowd at auction, they cannot tell that is is not new or as good as new or even not quite as good as new.
I must admit, every firearm I own was bought in a pawn shop. I just wont pay retail or even market value used for any firearm.