It's a half-pad for an English saddle designed to absorb shock and provide pressure-relief. You can read about similar designs at these two links: Equigel® Cushion Gel Pad | Dover Saddlery Ovation™ Comfort Gel Pad - Standard and Half Pads | EQUESTRIAN COLLECTIONS.COM
Yours is probably worth less, not only because it's used, but it looks like your isn't gel, but probably a type of memory foam. Look for some of these used on eBay
to get an idea of what they go for used. I've seen the Ovation gel on eBay for about $35 on the low end, and the Equigel for as low as about $30 used (very used). It looks like it's in nice shape. I would maybe try to get $30 for it?