https://royals.ms/forum/threads/s-129-15-2-slots-stonetooth-12-wa-fs.216606/ This ST is going for 6b-7b. That should give you an idea on what you can sell yours for. Its not something I can afford though. Good luck!
I'd offer direct 8b on it, please let me know if you want to sell it. As recent price of 141ST goes for 16b, which is your 131/2 ST potential to be with 10% and WS, I'd say 131/2 ST price should be 16b - 8b (cost of ws for 2 slots) which is 8b. Or with 2 x 100% event scroll which directly can be a completed 135ST, same price 8b (https://royals.ms/forum/threads/stonetooth-135att.212106/#post-1263120)
Open - S> 15/131 ST +5 | 2 Slots Available | MapleRoyals I opened a thread, it is up for auction. I considered your direct bid as a c/o and will start the bid at 8b.