My OTTB won't ride on grass?

Perhaps ask the owner to send you a video of her riding her in the field so you can see exactly what she's doing and possibly ask someone with experience what they think.

There could be a number of reasons why she does this...It's possible that she has been held up tightly in the field and it has resulted in her becoming agitated and fighting, that or it's also possible there's a soundness issue there that becomes apparent on firm ground and the owner is either unaware of or hasn't mentioned it to you. If she's unshod that could also be part of issue, it's not to say TBs can't go without but some do not have the best feet.

Don't buy this horse without having her fully vetted anyway.

ADD: Sorry yahoo is being a bit funny. I wouldn't wait until you buy her to see if there's an issue. It's well worth having her vetted and ruling out any problems. Bare in mind that lameness could end up costing you a lot of money or at the worst a horse you can't ride. Don't just take her owner's word either.

She seemed perfectly sound when walking on the gravel, concrete and walking through grass on the lead. Grant it this was only at a walk but she showed no signs of stress or pain on either footing. But I will ask to see if she has video of this behavior. When I get her, I may put her on a longe line and see how she responds. If she is not lame then I will know it's probably just behavioral.

The seller has had the vet out numerous times to check if there is anything wrong, and there isn't. She showed me the papers the vet gave her saying that she sound, and healthy in every way (From teeth to tail) but does require a chiropractor visit every six months or so.