You should certainly be careful insofar as you need to make sure that all the assets you use fit together in terms of style, quality and so on. And as others have said, using Marketplace assets for some things will be more acceptable than using them for others. As a general rule of thumb, the more generic an object is, the more easily you can get away with just using a marketplace asset. For example, fire hydrants, street lamps, cars, generic medieval swords, plants, that sort of stuff is going to look pretty much the same no matter whether you custom-make it for your game or buy it on the Marketplace. But more specific stuff like fantasy architecture, character models (especially main characters), hero props (like, if you’re doing an Indiana Jones-type game that revolves around finding an artefact, you probably shouldn’t use a Marketplace asset for that artefact), etc. you should make yourself (or have someone else custom-make for you if you can’t create it on your own).