If you feel like the person is asking too much or isn't going to be reasonable, then you just tell them "I'm sorry but I don't want to insult you". Or tell them, you're just going to be too far apart on price. If they insist on an offer, then give them one. The worst situations are always when the seller says to make an offer. Usually that is a red flag that they want crazy money (sometimes the opposite however) and you're probably going to offend them with the offer, even if they say they won't be offended.
Of course, you always want to give it some kind of effort to buy the books.
In most likely, you're going to deal with 2 situations.
1. You explain to them what the comics are worth (use eBay and Overstreet if needed) and they might be cool and actually listen to reason. Don't use Overstreet for newer comics, it shows a minimum value of $2-3 for most comics, which makes people think that their 90s comics are worth that, when most are barely worth 50 cents.
2. You will tell the person that the comics aren't worth that much and you can rant for about 20-30 minutes about why they aren't, giving various examples and resources, but they won't listen and no matter what you say they don't want to hear it. This one is alot more likely.
When people tell me how "old" the comics are, I usually use the analogy of relating it to a person's age. Someone born in the 90s or even 80s isn't all that old. Someone born the 70s is alittle older but not really old. Someone in the 60s is alittler older, but not super old. Most people will agree that someone born in the 30s and 40s is actually old.
No offense to our older boardies.
I find that sometimes when you engage in small talk with a person prior to making an offer, it can help them feel easier and build a trust with you, so when you do actually tell them what the books are worth, they won't be offended and might actually sell them to you. I got someone to sell me a Hulk 181 and ASM 129 for very reasonable prices a couple of weeks ago after alot of small talk. This same person thought Conan #1 was worth $7000.