Really an odd question:
Is 30 a 12?
Well I ski a 27, and have a size 10 foot. So by that logic a size 29 would be a 12. So your boots are a size 13. But I have a tight fit, you might have a looser one, so it could be a 12.
As for 12 or 12.5...well that makes no difference as 30 and 30.5 is the same shell. The difference is the 30s liner is sewn shorter...but that makes no difference as the liners stretch to your foot anyway....
If the boot fits wear it.
However, it sounds like someone is trying to buy them without trying them on...all you can do is relate..."I wear a size 12 Addidas, and these fit"...or what have you....good luck.