I made precooked frozen meatballs in the crockpot with jelly and honey BBQ sauce. It was really yummy and was wondering if the sauce that's leftover would be ok to marinate some ribs in?
The general rule is that if the food has not been exposed to potentially dangerous temperatures (40 - 140 F, 4 - 60 C) for more than 4 hours cumulative over its lifetime them it is safe to use. This would apply to your sauce, so if it was hot the entire time it was being used, it should be okay.
If your marinade meets that constraint, then you might consider reusing it. However, it will have been diluted and infused by the meatball flavors, so the result may be strange or it may be fantastic, depending on how those flavors interact.
Given that ribs are not inexpensive items, I would start with a new "marinade" sauce, so that you have consistent and predictable results.
As various comments have said, it somewhat depends on what you're going to do. Generally speaking, unless the marinade is to be immediately cooked with some other food, I'd say to boil the marinade first before reusing. That's a standard step recommended for marinades from raw meat or poultry.
Now, you might say, "But the meat wasn't raw; it was precooked." The problem is that even cooked meats can provide a growth medium for bacteria. Errors in cooling procedures or contamination after initial cooking could create problems. (For example, see this question on why precooked commercial foods often contain warnings to reheat to a minimum temperature before eating.)
Thus, my tendency would be to err on the side of caution and boil the marinade before reuse.
Note that this advice assumes you have employed the standard practice of marinating in the refrigerator. Marinating at room temperature -- even with precooked meat -- creates the possibility of significantly greater bacterial growth over time if any contamination has occurred. At some point you may begin to grow stuff that could even have persistent toxins that can't be destroyed by boiling. (Note that this might not happen for many hours or even days at room temperature, but it's hard to predict if contamination occurs. And while various elements of a marinade -- like acid or salt -- could inhibit bacterial growth somewhat, liquid around meat is a pretty good growth environment.)
Thus, to be absolutely safe, I'd personally boil the marinade before reuse. And if it sat at room temperature for any significant amount of time, I'd probably not reuse it. Yes, you might be safe without these precautions, but it's tough to estimate the possible risks.
EDIT: Thinking about this more, I'd say this is one the more "overcautious" food safety answers I've written here. In a whole lot of scenarios, it's probably going to be perfectly okay to reuse the marinade even without the precautions. But if you really want to minimize risk, this kind of question falls into what I think of as the "too many variables" scenario. How was the meat initially cooked? How was it cooled? How was it stored? Were the any opportunities for contamination after cooking? How was the meat stored during marinating? What are the contents of the marinade and how much might they inhibit (or promote) bacterial growth? Without firm answers to all of these things, I'd default to standard practice for marinade reuse with meat, as outlined above.