If by some chance the engine is not the source of your problem then look up "small engine repair" in the yellow pages and if it still runs well then maybe one of them might want it. If not, you'll be damn-lucky to get any money at all in return. If the engine is shot then it wouldn't hurt to call a scrap metal dealer on the outside chance they'd give a few bucks for it, but I wouldn't hold my breath. Otherwise, it's headed for the junkyard.
In Houston we always give things away to an organization called The Jesus Center. They'll take almost anything off our hands. They're great....they never call or solicit anything. You call 'em and they drop by and pick it up (and you get a tax deduction receipt).
I'm sure there is an equivalent organization in the Dallas area. Or call the Jesus Center in Houston and see if they have an affiliate in Dallas. The number in Houston is 281-590-3216
I don't know how it is in Dallas, but in Austin I set stuff like that out by my sidewalk, and the junk fairy usually hauls it off in the middle of the night. I've had people take much less desireable items than a dead lawnmower.
If it was a Lawn-Boy, you can sell it on Ebay and get some money. Those mowers have a nearly infinite lifespan, and folks who know how to work on them constantly troll Ebay for classic models.
Lots of home improvement stores have a program where you can trade it in on an electric model. I had an electric mower that I ended up giving away (I hired someone to mow) and it has performed admirably for 5 years.
Thanks to all for the advice...but I think this sucker is heading for lawnmower heaven. The engine still runs...the problem is the gas tank, it's got a hole in it and has separated itself from the engine. This thing has been repaired with bailing wire and duct tape too many times before to even worry about fixing.
I think I've found a dump by my house that will take it...and if not, I'll try the local high schools.