i've just sold a few things on preloved, and a friend has just sold her saddle on their too. there's a number of local horse groups popping up on facebook and the one i'm a member of constantly has stuff up for sale and seems to shift pretty sharpish - might be worth a try. bonus in that the people are generally local so no hassle of packaging and getting to a post office - most seem to be meeting at yards/ convenient(ish) locations for both parties
Mine is called something like the Bristol and South West Horse Information group, pretty sure i've seen a similar one for Plymouth/ Devon area. People use them for all sorts of stuff but advertising stuff/ horses for sale too.