I believe you are asking about building yourself vs having someone else build one for you. Most 4 and 5 star recipes require a leveled-up lower-star item. Some people will build them for sale, however, by making the earlier models, leveling the earlier, and then crafting it. This is more common with 5* gear, though, since they can get a better return for their time investment. So if you look, you can indeed find people who have higher level gear for sale (also, a few items, Stone Tortoise Shield, for example, that are 4* or higher that have no earlier alchemy path, so are more easily made/sold to you.

As for building vs buying, it completely depends on your monetary situation (whether you can afford it) and time investment. A 5* item typically sells from 2-3k CE, or ~100-150,000 crowns, higher for decent UVs or hard-to-get items. If you don't have or want to pay for recipes, this is not that bad a deal, as just getting the recipe will hit you for 60-70k crowns, then crafting even more, plus mat costs if you don't have some...

By the way, don't give money to people to commission items unless you really trust them. Scammers exist, and if you agree to a trade and the other person flakes out on you, the GMs are unlikely to do anything about it. And a 3* item for 5k is really questionable, since it costs more than that just to make the item,. Unless they're just UV spamming or building out of mist energy, in which case it should be pre-made.