If the real sin behind the marks the beast is simple everyday economic buying and selling, then everyone on this board is already guilty of it.
Think about the logic behind it. The Mark will be used to control buying and selling. The only way to continue buying and selling will be to take the Beast's Mark. The Mark will be a sign of loyalty to the Beast. Will it be a "real sin" to buy and sell at that time? Sure it will be. The logic is simple when you choose to see it for what it's actually saying.
But what about buying/selling as a sin at THIS time, before the Mark? You seem to be hung up on the concept that, because you've depended on buying and selling for your whole life, there is no problem with it. It's common, normal, ordinary..."simple everyday economics". And if it's "simple everyday economics" now, why should it be any different when the Beast comes to power? You've reasoned the issue on the basis of, "but that's the way we've always done it".
That could be true, if only it weren't for Jesus' teachings on the same topic. The "real sin" isn't just doing what's "normal". The real sin is in choosing to believe it's normal to act contrary to the values of Heaven. Consider this evidence...
LK 17:26 And as it was in the days of Noe, so shall it be also in the days of the Son of man.
LK 17:27 They did
eat, they
drank, they
married wives, they were
given in marriage, until the day that Noe entered into the ark, and the flood came, and destroyed them all.
LK 17:28 Likewise also as it was in the days of Lot; they did
eat, they
drank, they
bought, they
sold, they
planted, they
builded;
LK 17:29 But the same day that Lot went out of Sodom it rained fire and brimstone from heaven, and destroyed them all.
LK 17:30 Even thus shall it be in the day when the Son of man is revealed.
All normal, ordinary, "simple everyday" activities and yet Jesus criticized them. Why? Did he really think that eating, drinking, marriage, planting and building were sinful activities? No. The problem is that the people were just too busy with the "cares of this world" to care about what God wanted. He said it will be the same in the end time and isn't that exactly what you've described with your "simple everyday economics" doctrine?
It's like that with our dependence on mammon (money and the things money can buy). We've come to believe so fully that money is the source of life that we give nearly all of our time working to get more of it. Most of us genuinely believe that we'd die without money nor can we see any other means of living which is not dependent on money. The spell is so strong that we can't even hear the son of God say anything different. We argue and explain away any teachings of his which are contrary to working for money.
Consider this other evidence:
LK 14:17 And sent his servant at supper time to say to them that were bidden, Come; for all things are now ready.
LK 14:18 And they all with one consent
began to make excuse. The first said unto him, I have
bought a piece of ground, and I must needs go and see it: I pray thee have me excused.
LK 14:19 And another said, I have
bought five yoke of oxen, and I go to prove them: I pray thee have me excused.
LK 14:20 And another said, I have
married a wife, and therefore I cannot come.
LK 14:21 So that servant came, and shewed his lord these things. Then the master of the house being angry said to his servant, Go out quickly into the streets and lanes of the city, and bring in hither the poor, and the maimed, and the halt, and the blind.
Buying, selling, marriage, merchandise...all everyday ordinary things, and yet they still managed to stop people from acting when God called on them. Can't you see it?
An unwillingness to change. An unwillingness to forsake. An unwillingness to hear. Anything becomes a "real sin" if we put it before God, and that's exactly what our dependence on mammon represents.
This is why the AC will use money as his "mark", or sign of loyalty and why the Mark will be so completely effective. If it were jelly beans the people of the world felt they could not live without then he'd use jelly beans as his mark. It just so happens that it is not jelly beans which they feel they cannot live without; it is mammon (money and the things money can buy). The point isn't so much about getting people to follow the Beast as it is about getting them to STOP following God.