Question about melding materia

I've melded a strength materia to a weapon before, but on the weapon by the materia it said I was getting +0. Is this just specific to me? Like if I sell this to someone else is there a chance they would get the bonus? What is actually going on here, and how can you tell whether you will get the stat bonus or not? I'm a paladin looking to meld materia to my lvl 33-35 armor and weapon, so I'm probably looking to get parry and vitality or skillspeed, but I have no idea how to tell if I will actually get the benefit from these materia, and the active help didn't explain any of this.

Nope, all items have a limit for each stat that can be melded. e.g. +3 str, then all +str after that will be +0. The limit varies with item level and how many of a stat is already on the item but there is always a limit.
On the melding screen, it will show what the effect is going to be. If the effect is going to be +0, I would not meld it, unless you are using "junk" materia melds for faster spirit bonding.

If the number is in red, it means the materia will try to raise the stat above the item's stat cap. It then lists the amount of a boost you will actually get from using that materia. If it lists +0, that means the item is already at the cap for that stat.

Generally speaking, HQ items (non-accessories) are always at the stat cap for their main stats (primary stat (str/dex/int/mind) + vitality). Materia slots are really for secondary stats, especially ones that the item doesn't already have. For instance, if the item has +strength, +vitality, +determination, and +skill speed, you will likely get no benefit from strength or vitality materia and only a small amount of determination and skill speed materia will help. However, you'll get a really nice boost from, say, critical hit or accuracy materia.

Thanks! I thought I had looked the last time I melded to see if it said what the bonus would be but couldn't find it. I'll just have to look harder next time. This should give me a pretty good idea of what materia I need to meld to what.