Read the article slut! I'm trying to help you here. Trundle bed is way more exotic, you'd get more for it! Trundle beds are worth like $7-8,000. Day beds are only worth like $3-4 at the most
From wikipedia: "The lower bed on some trundle beds can "pop-up" to be the same height as the bed they are stored beneath, creating a larger bed that may be used for guests." (please note your original description, your second wiki link makes no mention of this amazing feature)
But I guess I'll give you it goes on to say: "Often used as daybeds..." ...but since it's under the definition of Trundle Bed, I think you need to respect the bigger picture here. If you were selling a Ferrari, you wouldn't say, 'for sale: red car' would you!!?!?!? C'mon, would you!!?!??