I agree that it's (albeit unintentionally here) awkward if you canvas interest for something before stating what the full offered deal is. "Could you use" is only one side of the bargain without stating what it would cost them and does sound like you're just wanting to get rid of it rather than selling it.
I'm not saying it was your deliberate plan at all, OP, but it happens to be the same principle that a lot of salespeople use, especially dodgy ones - establish that the potential customer IS interested and only then spring an unexpectedly high price on them.
I know you've acknowledged that you maybe didn't word it as well as you could have done. They're under no obligation to do so, but as a PP said, most caring parents, on learning that you weren't able to use them (as opposed to not really your cup of tea) because of the child(ren), would have said "Well, we could use them, but how about we babysit for you so that you can enjoy your gift?"
I very much disagree with those protesting that you got them free, so how dare you want money for them. If you'd won them in a competition or been handed them as a perk of working in the industry, I'd agree; but when they were your Christmas/birthday/anniversary GIFT, that's not the same thing at all.
This reminds me of the recent 'gift experience voucher' thread, where people were insisting the same thing. If somebody has actively chosen you to receive a gift that they've specifically bought for you, then it's not unreasonable to want to get some personal benefit from it. Looking at it coldly, you might well have reciprocated with the giver and given them something of equal value for their enjoyment.