Here is what the header should look like when accepting an offer:
{
"rootTx": "",
"action": "acceptoffer",
"timestamp": number,
"data": {
},
"signature": "SHA3-512 hash of signature of rootTx + action + timestamp + data"
}
This is what the data field should look like:
"data": {
"txAccept": "" // The transaction that the offer was created in
}