I tend to agree about just accepting the gift graciously. However, if you suspect your neighbour may not know that the item he is gifting has some value and you don’t want to risk hard feelings down the road you could always say something like “ hey, these things can fetch a few dollars you know? Are you sure about this or can I give you something for it…?”. If he still insists on gifting it, great, but at least you’ve given the option and you know both parties have the same amount of information going into the deal.