Legal Question in Intellectual Property in Kentucky

I understand your question to be "Can I buy generic helmets and put NFL or NCAA logos on them and resell them?" The answer is not legally and not without risk.

It is legal to recondition or repair trademarked goods and resell them. For those, the trademark owner had control (or a fair opportunity to exercise control) over the quality of the product bearing its trademark and the original sale gave the trademark owner its royalty, if any, and that protects their quality control interest and "exhausts" their royalty interest.

If, on the other hand, you buy new stuff and put a trademark decal on it, you rather than the trademark owner, are determining the quality of goods bearing their trademark and the trademark owner has not received their fair royalty, except perhaps on the decal. That is why the law makes unauthorized new sales of trademarked goods (i.e. "counterfeits") infringement and unauthorized resales of used legitimate items not infringement.

So, the answer to your question depends on what you mean when you say "I want to do this." If this means exactly what the "few sellers" on EBay do (sales of reconditioned original equipment) YES you can. If this means counterfeit new items, NO you can't, at least not without serious legal risk. Recognize that some counterfeits may appear on EBay until stopped, so you were wise to ask your question so you stay legal and avoid trouble.