Just to be clear, it is not illegal to bring more than $10,000 into Mexico from the US. You must declare on boths sides of the border if you have $10 K or more in cash or negotiable securities, like a check. I know of several instances when this occurred with no problems. How you citizenship will effect things I do not know. You say the total is just over $10K. If the seller agrees make a contract and make two or three payments. You don't have to do it all at one time. You could also get your withdrawl limit raised and take some out at a bank cajero. You could also purchase prepaid debit cards and bring some that way. Cash, cajero, debit cards, split it 3 ways, just not $10 K or more per trip or declare it. There is a NBA in Ajo if you deal with them. Do it soon because there are some changes coming about in how the money must change hands in a real estate transaction. Yes a Notorio must process the transfer of the property and determine any taxes due and check for ownership, but they will probably not want to touch the money, if they do Hacienda comes sniffing around. Buyer and seller sign when all is paid as agreed between them. Soon a requirement will prohibit cash transactions and the money will have to go through a Mex. bank and YES the bank will charge for it. Dee Brooks at Twin Dolphins will help is you need it.
Not sure about Safe Send but all other methods of money transfer have limitations on total amount per transfer and number of transfers over a time period, like a month.