Hi @drakanstyr I think the best way to sell them would be by auctioning them on eBay, but be sure to start bidding at a price you'd be happy to sell them for. For example, if the voucher are worth £20, it would be risky starting bidding at £0.99 because there's always that chance that only one person would happen to see it and bid in time before auction ends- and the sold price would be a sad £0.99. So start at a price you're comfortable with.
To be honest, with vouchers, I've seen people on ebay put them up for £2-£3 less than the actual voucher price- so perhaps you could try that. You could also list it as "buy it now" instead of auction if that appeals to you more. I would try eBay first myself. Good luck! 🙂
P.S. If you want to try other avenues, you could list on eBay, gumtree and the newer site known as "Shpock" - the downside with those sites is sadly, you do get a lot of time wasters, whereas on eBay it's listed and once the sale happens, it's a done deal.