Since I mostly purchase complete sets (except for vintage), I've experienced the receiving incomplete sets many times, but I've always had the seller fix the problem. Except for factory sealed sets, the buyer should also expect their "complete" sets to be complete. It is not okay for a seller to expect you to just take it. It is their responsibility to either complete the set for you, or to refund you the amount it costs you to complete the set. I would be leaving negative feedback if a seller doesn't do this. Check your hand-collated sets immediately, and let the seller know if you find something wrong. Most of the time, I believe it's just an accident. It doesn't matter. Even if their advertisement says "as is", as long as the product is advertised as a complete set, you have the right to expect a complete set. As a seller myself, I'd either purchase the missing card myself and have it shipped to your house, or pay you what it costs you to replace the item. That's what I would also expect as a buyer, and I hope you do too.
2 Examples:
I purchased a 1928 Tharp's Ice Cream (F50) reprint set of 60 from a guy in Chicago, and the set was missing a Chicago White Sox player. This didn't seem like an accidental oversight to me, but the seller didn't have a replacement, and refunded me s commons for this set are not easily found. The seller refunded me $10 from the set, and I ended up finding a replacement about a year later for about $10.
I purchased a Garbage Pail Kids collection of All-New Series #1-7 sets consisting of about 600 cards. If you are familiar with the numbering, it goes #1a, #1b, #2a, #2b, etc. Well, I had a duplicate #2a with no #2b for the All-New Series 4 set. The seller had an "as is" disclaimer on their ad, but it said that the sets were complete. I searched for the cheapest replacement on Ebay, which was $5, and had him refund me that amount when he couldn't produce the card. I said that was what it would take to satisfy this transaction. Never discuss, or use the word, feedback during communications with a seller if there is a possibility that you may have to leave anything other than positive feedback. Ebay reverses ratings a lot based on your communications with the seller leading up to any bad marks.
Don't let the possibility of getting a incomplete set discourage you from collecting this way. Like I said, I've always been able to get the seller to remedy any mistakes. Just be diligent in verifying your purchase.