First of all, creating tokens and placing them on NFT trading platforms has a commission, and in most cases it is easier to buy someone else's token for $10 than to make your own tokens.
For expensive token sales most of the associates costs are the platform fees. For OpenSea it is pretty much the sales fee which starts from 2.5%, regardless of whether it is a new token or an existing one. For an existing token there is also the gas fee, but it is negligible compared to the sales fee, especially if the token is placed on a cheap network.
Selling new tokens is always suspicious, because your tokens will have nothing (ecosystem, project and so on). If you are a famous artist or designer, I agree with you, but in other cases I would use ready-made NFT projects.
An existing NFT already has a market price which in this case should be low (e.g. $10). So there is a sudden jump in the price that will look suspicious. It would be even more suspicious if it is a cheap token that is mass-produced (and cheap tokens are mass-produced quite often), because in this case there is an established low market price for this token.
A new NFT on the other hand does not have a price, so there is no need to explain any big price change.
Then again, in most cases the government would just take the taxes and not bother to ask for too many questions. As long as it is money earned through legal means (not from drugs, arms traffic, or any other crimes) and the person just wants to pay the taxes and put the money in his bank account, it would probably be OK, regardless of the NFT type.