When i recently sold my quality alrounder/potential low level eventer i sold him through Horsemart. I had him up in as many places as i could and only got enquiries from Horsemart. I found Horsequest quite expensive for an internet site and had no enquries from it!
I think it very much depends on the horse you are selling as on Horsequest there is nothing really under £4k
and they all seem to be competition horses where as Horsemart seems to be more about alrounder horses and prices range from around £2.5k-£5K
Good luck though!
Just advertised my girl on Adhorse - although half-hoping I'll get no calls!
When I last sold one I put him on Adhorse; nothing - then Horsemart; muppets only - and finally Horsequest; sold within a week to a fantastic home, had to get the ad pulled because the phone never stopped ringing!
Having used most places over the years, horsemart brings in the most enquiries; in the last few weeks we have sold all four horses via there we advertised, and it's also so cheap.. (sorry H and H but you aren't...)