The lifetime warranty wouldn't count for much, especially if you have to pay postage to RMA. It's kind of like selling a house with a pool, it's nice but doesn't factor in to the selling price no matter how much the owner wants it to.
If your shopping old DDR2 memory I doubt you care too much about the speed either so I wouldn't pay more for 8500 vs 6400 I'd just take the better one if it's the same price more or less. So $60 for a new set + postage $65, I'd be offering about $35 for your best DDR2 2x2GB, at a stretch I'd go $40-45.
And you really can't compare eBay prices, they go from ridiculously cheap to overpriced. Several of the DDR2 sets have shipping charges of over $40, ridiculous.
In case your wondering, I do adhere to my own values, I just sold a 2x1GB DDR 6400 Mushkin set for $10 and some idiot has them on ebay for $88 + $20 shipping.
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