I remember reading that Ploni Almoni was Elimelech's brother. Note that the laws of inheritance are such that Naomi could not have inherited the property of Elimelech. Since his children were dead, the land would then go back to Elimelech's brother (Ploni Almoni) and then down through that line. In Kesuvos 104, we see that a woman can arrange to have her husband's property sold to pay for the kesuvah. Naomi did not wish to do this for as long as she could avoid it. The implication as well is that Boaz arranged this beforehand so that he could then insist that the redeemer also marry Rus.
Since she was selling the land to cover the kesuvah, the actual redemption and the marriage to Rus was "lifnim mishuras hadin". Only the brothers of the dead husband could be considered in the category of Yibum. As explained by RAMBAM, since none of Elimelech's children were alive, Rus was not subject to the halachos of yibum and could have married anyone she wanted (or who would have her).
There are meforshim who say that Ploni Almoni refused to marry Rus because he knew that there would be a dispute. He considered that she would have to marry a gadol (like Boaz) so that the psak of "Moavi vlo Moaviah" (a female Moabite could convert and be accepted) would be accepted in future generations. Indeed, Dovid was originally rejected by people who did not want to accept that psak and Shmuel Hanavi wrote the book of Rus to emphasize the ruling.