Would it be faster to do relative seeks?

It would, but I think almost all modern implementations of seek would automatically translate an absolute seek into a relative seek anyway. I don't know for sure, but I find it hard to believe that an absolute seek would ever cause the OS to 'go back to the beginning of the file'.

In reality, seeking simply adjusts a number somwhere and it's not until the next read or write that it has any affect at all.

The third parameter to seek is simply a throwback to when files stored on tape were commonplace.

With regard to the possibility that the file has fixed length records, you're right it would be much easier and faster if that were the case. But from the (scant) information provided by the OP, it seems unlikely. Whilst he shows the serial number padded with leading zeros, he also shows a name field, which is empty in the given example, but names vary in length, so unless they are all empty, it probably indicates that these are variable length records.