Gun Control & RKBA

The same rules for invalid buyers apply to both dealers and private sellers, regardless of location.

Licensed dealers always have to do a NICS check as part of their buyer verification process.

Private sellers are prohibited from using the NICS check as part of their verification process. Any sale to an invalid buyer can be prosecuted if some type of verification was not done. Requiring the buyer to have a concealed carry license is a common verification since it shows that the state did the background check. Selling to friends/family is "safe" because you know their backgrounds. Many people will sell to strangers with no checks because the vast majority of the population are legal buyers, just playing the odds usually works.

To answer your question, a private sale may not need a straw purchaser if there is foolishly no check being done by the seller. The result is a simple illegal sale to an invalid buyer. If a straw purchase is used, you have the illegal purchase by the straw purchaser plus the illegal sale by the straw purchaser to the invalid buyer. If any of that was to be prosecuted, it is up to the DA. Unfortunately, many (most?) times it is not prosecuted.