I personally try to buy used on the forums or on eBay. Depending on what model the knife is, and the seller, used can mean it was taken out of the box and looked at, all the way up to they chopped concrete with it and then reprofiled the edge. As long as it is not abused, if it is a model I am looking for, I will prefer to get it used. Worn in, so to speak, and I don't have to pay MSRP.
MSRP is tricky. I know some vendors pay WELL below MSRP from their distributors (how much does a company make off their knives?). Depending on what overhead they have, the price you pay the dealer could be MSRP or above (I have seen that in several places), or a good chunk off MSRP. Online dealers usually offer better deals than brick and mortar, but you don't have the knife right in front of you, and sometimes eBay sellers are selling stock that they don't have.
So, I bought a BK7 for a hundred bucks at Country Knives. MSRP is well below a 100 bucks. If I left it in the box, and never used it, I probably wouldn't be able to get a hundred bucks for it. But a dealer, who has to make up for what they have spent, might stand firm on selling it for a hundred bucks. I have traded knives at gun shows, that were obviously used, but included the box in the trade, and came back later and the dealers had put the knives on the table as new in box. Charging close to MSRP prices. Not all dealers, but a few.
With Sebbies, it depends if you get someone who is a collector or a user, as said. Personally, I am a user. I amass knives, but I don't collect them in the general definition of the word. Thats why, with knives, unless they are a collector item, or a short/limited run etc, you probably won't make what you spent on them.