Concur. Previous owner likely half-assed a MP DIY replacement back to stock to sell the part off, and left you with the results, either by not properly tightening the bolts (they came loose) or by simply not putting them back in at all. Did you buy directly from owner or through a dealer? If the former, I would contact this person with questions. If the latter, then it is what it is - get some new bolts put in. Regarding the undercarriage tray fasteners, they have a known history of corrosion - I don't have the thread handy, but there are some companies that sell a titanium replacement kit that includes all new hardware for this very case. You can do a search for that.