Depends a lot on what you're selling and your goal.
FB Marketplace is better for: physical items, local sales, buyers who are ready to purchase right now. The intent is high — people browse Marketplace the same way they browse eBay. Conversion tends to be faster but competition is visible and price pressure is real.
FB Groups are better for: building trust over time, selling digital products or services, niches where relationship matters (coaching, freelance, info products). A group post can get organic reach to thousands of targeted people if the group is active. You're also not competing on price the same way.
For BHW-style use cases (accounts, services, digital stuff), Groups in the right niche will outperform Marketplace because the audience is pre-qualified. Marketplace is more of a mass-market tool. The downside with groups is that good groups have strict rules about promotion, so you need to add value first before posting any offers.