eBay is more straightforward and safer on all counts with better customer service.
OfferUp is self-explanatory for local/in-person transactions, but if you intend to be a shipping seller, it is still in its infancy stage. If you intend to buy and opt to have the item shipped, Stripe (the payment processor the OU uses ) is not as efficient and effective as Paypal. As @goblin recommended, conduct a search within the forums in regards to everything involved when buying/selling using the shipping option.
If you intend to only "ship" as a buyer and/or as a seller, I would recommend Mercari over OfferUp, as it is more established.