Good Buy On LMC

Don't really know to tell you the truth. I've seen such sets at coin shows for more than that but all coins were really very BU. If you did order that set and it comes to you in circulated but AU, it's not exactly anything fantastic.
The odd thing is that most of such sets are from 1959 to present or 2008, pending on when finished. Why the 1958?
Also, at coin shows when I've seen these they are in a Whitman Folder meaning someone had to really press the coins into those. Usually with their thumbs so eventually a large thumb print will show up on the coins. If these coins are just loose, no 2x2's, no folder, what are they in? If in a nice Dansco or Whitman Album, then the Album is almost worth that. Are coins BU or MS? This might be a lot of BS you know.
If I were you I'd want to know more details.
As to the proof sets, also difficult to determine without more information. All in original cases? All in original packages? None opened?
What you might want to do is purchase a Red Book, 2009 edition is OK, pages 330, 331 have a list of Proof sets and their approximate values. These are in general about 20% or more overly stated in values so you could add up those sets and deduct about 20%.
Next is there a return policy? Do you have to pay for S&H? If returned, do you have to pay for that postage also? Do you have to pay for insurances? Are they requiring you to pay any sales taxes?
Do you know if the seller is a reputable dealer?
If on ebay, be carefull.

As Thad and I have discussed memorials alot lately, we both agree that most sets you would buy like this are very low MS grade. Memorials are tough. If you are looking for a quick fix and are not too picky, then these sets are just fine, but if you want the best examples you can find, this is not the right path. $25 is a pretty decent priced. These sets are thrown together from rolls with no regard to grade, so you get whats next in the stack. You can always buy the set, pick out a few of the bad apples and upgrade those. You might get some really nice ones, never really know!

As for the proofs. You could probably put together a set for cheaper, i'd say around $100 is feasible. Some of these sets will have spots/issues or toning that you could avoid buying separately.

I guess its all about how you want to collect them and how serious you get with cents. Good luck!