I've never knocked on a door, but I've haggled over the bed of a pickup truck in a grocery store parking lot. Based on that limited experience, I'd start by simply offering admiration for the grill that they've cared for all of these years. Take away any pressure for an immediate sale. Likely it has sentimental value, and if you leave some contact info they might be happy to someday pass it off to a kind person who they know will care for it.
...or they might be the greedy sort and ask $100 for it, right then and there! ;-)