Piecing the cards you need one or a few at a time can be expensive and time consuming. If you have a chance to get a bunch in one shot, then it is usually worth considering.
"Is that the best use for that money right now?" is the question I would ask. If you have the extra money and enjoy collecting enough to spend it on cards, then go for it.
Often those larger lots can be an even better deal, because most buyers don't need everything and spending the money on dupes isn't worth it to many. If you can flip the dupes, even better for you.
I bought a lot of 400+ 1978 Topps auto'd cards but I had a bunch already, maybe 1/3-1/2 of the lot. There were some really good ones I needed and a solid selection overall. However, because I love the set and don't mind dupes, I got a screaming deal overall. Those types of things will always have a decent resale value, so I felt good about pulling the trigger on the deal.