As the saying goes, “There is no such thing as a free lunch!”, and many buyers might have some doubts about its low price. Grammatically, you're fine (with the one correction). However, it makes me think you might still have a question about the meaning of the phrase "there's no such thing as a free lunch." It means that you shouldn't trust something that seems too good to be true because there is probably something you have to do to "pay" for it. For instance, there might be tickets for a tourist attraction at a very good price, but you find out when you try to buy them that you have to listen to a timeshare presentation to get them.

As the saying goes, “There is no such thing as a free lunch!”. Many buyers might have some doubt because of its low price. [The buyers won't have a doubt "about" the low price. They will have a doubt the "free lunch" (the thing they are getting or buying), "because" of its low price.]