Not only from the BATF--they involve the IRS too.
Good point. Referrals for undocumented businesses flow from BATFE to IRS.
However, info doesn't usually flow the other way. Unless criminal tax fraud is being alleged, the IRS Revenue Agent who audits your unlicensed gun-selling business does not care that the business is illegal as long as you report your income and pay your taxes.
Plenty of drug-dealers file their tax returns and pay their taxes (well, under-report and under-pay, I'm sure) just to protect themselves against the DEA piling on with a criminal tax fraud referral to the IRS. When they file, sometimes by literally sending their attorney to the local IRS office with a suitcase full of cash and a 1040 filled out for "John Doe", the civil side of IRS simply doesn't care about the illegal source of the funds. Of course, when things like that happen, the criminal side of the IRS often already knows about it and shows up at the same time. There's nothing stopping anyone from employing a similar (but hopefully proactive) strategy when selling guns.
My point is - If someone decides to engage in a business that might get them arrested by the BATFE, it would be a good idea to file and pay taxes from the beginning. The IRS won't care and they'll avoid a future criminal tax fraud prosecution.
What bothers me about all this and what makes it actually on-topic for this thread is that anyone who considers buying firearms for investment purposes actually has to think about this stuff. If you're paying a half-million for a Walker Colt that you intend to hold onto for 10 years and then sell for double that (you hope), then no one is going to accuse you of running an illegal firearms dealership. But if you buy a dozen hunting rifles a month for a couple of years, then start showing up at gun shows selling your oldest stock for a couple of hundred profit at roughly the same rate you bought them, then the real possibility exists that a potentially workable investment strategy will be viewed as an ongoing (unlicensed, illegal) business.
The original poster specified a 2 to 5 year time frame. I think my earlier comment about dancing on a wide, gray line may have been more insightful than I realized at the time.
So far, I like the suggestions of LEO-marked Colt ARs, basic Cooper rifles, and high-end shotguns.
The ARs have markings and people pay for markings. Have you ever noticed how a worthless, worn-out piece-o-crap rifle somehow becomes a magic dollar-attracting machine when it has a "Property of U.S." stamp of some sort on it? I once saw a table full of Gamo 126 sidelever air rifles at a gun show. They were beat to heck and that particular model has long-term maintenance issues, anyway. They might have been worth $20 each, maybe $30 if I really wanted one and had all the seals sitting at home to do the rebuild. The seller had them marked at $600 each and said he had no trouble selling them to collectors simply because they were ex-Army trainers and properly marked as such. Insanity. Profitable insanity, but insanity, nonetheless.
The Coopers are good quality without getting crazy and everybody recognizes that. Pedestrian specs make them more suitable to a broader range of buyers so they'd be good as relatively short-term investments. Long-term is a whole 'nother ballgame where the weird and rare often tend to bring a premium. Nylon 66s, for example, have gone up too far but have you tried to buy one of those (I can't remember the model number off the top of my head) single shot bolt action .22s in the Zytel stocks that Remington made back in the 60s? Given enough time, the big bucks are in the weird stuff.
High-end shotguns I agree with not just because of the value of hand-fitting but also because the shotgun guys always seem to have money. The same isn't true for the pistol guys, even though there's lots of hand-fitting on some pistols. I've never understood the demographics - shotgunners have money, pistoleros are broke, and the rifle guys are somewhere in between. There are plenty of exceptions to that rule but have you noticed how broadly applicable it is? In any event, you can find meticulously hand-fitted revolvers from Freedom Arms that sell for around $2K new but in barely used condition fall to $1600. Well-used (but still more accurate than a lot of new, off-the-shelf hunting rifles) FAs that would sell for well over $2K new can often be found for $1100-1200.
Personally, I tend to buy old single-shot XP-100s. While there are lots of choices for actions these days, when someone wants a bolt-action pistol, the XP tends to be the first thing they think of. I could probably sell any of my older acquisitions for 4 times what I paid.
But however much I think the OP has gotten some good answers, I feel the need to stress that the 2 to 5 year hold window he's specified is too short. If you buy a lot (definition of "a lot" completely dependent on how much coffee the BATFE had to drink this morning) of firearms with the intent of selling them in 2 years, IMO you're asking for trouble.
Hold for 10 years, hang onto the original paperwork so you can document the passage of time, and I don't think anyone can make a reasonable case against you.
Be careful, VanIB. You have a good idea but I'd hate to see it blow up in your face.