Selling access because is soon going to be the POTUS is very unethical. Simply throwing a New Years Eve party is not. The money is the difference. This is just another perfect example of how Trump is all about money and promoting himself. He isn't doing this because he cares about our country. He is doing this to advance himself. He cares about no one else.
It is absolutely wrong that individuals get the power to pay for access and time to politicians. It eats up the politician's time he should spend using governing like an objective professional and creates serious conflicts of interest that weakens his power and ability to govern in a fair manner.
No, it's not wrong for the president-elect to host a New Year's Eve party, because it's likely that he was able to raise some money for the Republican party. All politicians, no matter the level, sell access in order to raise funds. That's just how things are. It's not fair to expect the President to be any different.
People opposed to Donald Trump being elected president are attacking everything he does, regardless of whether or not there is a legitimate complaint. President Obama has done many things as bad or worse than Trump's actions without having any complaints. The Clintons are, and always will be, notorious for marginally ethical behavior. Trump is just more closely scrutinized.