As long as they don't commingle inventory it's relatively easy to avoid (not sure if that's the case or not, but seems like most of the 3rd party sellers do their own shipping)

FWIW my main annoyance with Walmarts website is that it's not clear if you package is coming via shipping service like FedEx, who has access to my apartment complex, or just some dude in his car who needs to call me while I'm at work to be buzzed in

Target does allow third-party sellers on Target.com (and the app), but they allow in-store returns on anything — even third-party items. When I worked as a receiver, there were random items to process out that would normally/likely be salvage, but went to the returns processing center because they were from online orders. There also seems to be curation.