Apart from being properly documented, It's only important if it changes between packets or between different users using this code.
Your pasted code doesn't change the afore mentioned packet section (more precisely this part: 0xA8, 0x00, 0x50, 0x39).
I don't care about, for example, 0x00, 0x00, 0x00, 0x00, even though it's also undocumented, simply because it's always the same value. Change user or number of packets (or number of items in packet, or quantity), value remains the same. It may be unknown, but it's also irrelevant to the use (at least up to the point of normal, intended usage of the packet).
Back to the unknown 0xA8, 0x00, 0x50, 0x39 part, it's not related to items being sold, for if it was, it would be a part of item struct at the end of packet.
If it works consistently for you, after several relogins AND with different characters, it would also seem it's not related to any character or shop data.
For additional info, I add my sample (same endian as yours):
0xB2, 0x00, 0xC8, 0x99
This was obtained on one purchase with one character on one shop. I will get more samples, from different shops using different characters, for comparison.
Edit:
I've checked value at my end after restarting the game and it's still the same.
Managed to track it to player struct, now I need confirmation from you Interest.
Please check the following offsets on your end:
PartOne = [[Base + 0x34] + 0x08 ( 2 bytes)
PartTwo = [[Base + 0x34] + 0x0A ( 2 bytes)
They should be:
PartOne = 0x39, 0x50 (reverse endian from network packets)
PartTwo = 0x00, 0xA8 (reverse endian from network packets)
Put together like shown below (and network packet endian), they should form your value:
PartTwo, PartOne = 0xA8, 0x00, 0x50, 0x39
Alternative offsets are also:
PartOne = [[Base + 0x34] + 0x7B4 (2 bytes)
PartTwo = [[Base + 0x34] + 0x7B6 (2 bytes)
If you could please confirm or deny this, it would help.
Thanks.