The indication that the seals are bad is negatives that have fogging along the right hand side if you view them with the images upright, and with the base side facing you.
Leaking seals allow light in through the dark slide slot.
I have three backs, and have owned them for roughly 10 ish + years, only one has needed a seal replacement, so far, knock on wood. If you only have one back, and don't have a need to remove it to change the finder, for example, then storing it without the dark slide will help keep the seals from getting compressed.
The last time I bought seals, I bought them directly from Hasselblad, because it wasn't obvious which ebay sellers had a good product. The cost was slightly more, but only slightly.
Oh, and welcome to APUG/Photrio!