Bought A 1973 Proof Set However

It is probably the least important coin in the set and it is still a proof. A proof is a proof. If they advertised the whole set as deep cameo and paid the price for such a set then you have a reason to send it back but the 1973 proof set is an inexpensive one and you didn't overpay. Actually, if I had a 1973 S nickel that looked like it was a BU coin, well, I would probably like it for being different. I don't think it is a big deal and certainly not worth complaining about.
I'd like to see a picture of this set and the nickel. Don't leave bad feedback for this, the seller probably didn't even consider it. Also, look at the glass as better than half full with all the other coins being "gorgeous".

It's not uncommon for proof sets to have a substandard coin or two. Even when they were brand new there would be some bad ones and over the years the numbers just go up and up. It's these clunkers that tend to end upin circulation when wholesalers bust up the sets.

Generally I wouldn't worry about it unless it's one of the valuable coins in the set. Even then though if you buy several you might get a bad one. Anywhere from about 1% to 6% of the coins are bad depending on denomination and mint. Some of the early zinc cents might be over this by now and these are expensive coins.