Chuck,
I'm not sure about the 5hp Pony prices, but I just bought a 1988ish 8hp Horse on eBay for $1000, the deal included the tiller, the hiller/furrower, a snow blade, wheel weights, tire-chains, and a wood
chipper. Yeah, I got a GREAT deal! /forums/images/graemlins/laugh.gif
I've noticed the new Pony (7hp) models on TroyBilt's web site are priced starting at $999. The smaller framed ones around 6hp are from about $500-$750. You'll have to figure out what is most comparable. I don't think the additional HP in the new ones gets you much, the gearing will allow your 5hp to do all you can ask of the Pony.
I'd say you should probably expect to pay about 40-50% of new if it is in good shape, maybe a little less if not. Has it been stored indoors or outside? Is there much rust? Does it run well?
As to it jumping out of forward, you may just be able to adjust the forward detent stop-block, if that doesn't solve the problem, they are available for purchase from customer service as a replacable wear item. If I had it apart and was in there anyway, I might also replace the belt and the reverse disk (rubber ring on a steel disk) just to save myself the hassle of tearing it apart again for a while.
Good luck with your deal. That old Pony should serve you well for many more years. /forums/images/graemlins/smile.gif