Two people propose to you with rings that cost 25k each. One ring is resellable for 20k. One is resellable for 5k. Which one do you think is more likely to renege?
http://en.wikipedia.org/wiki/Engagement_ring#Legal_ownership
Since you'll have to decline one of these proposals, one of them gets their ring back. Which one are you more likely to accept? All things equal, the 20k resale ring.