I was checking out some Lego in John Lewis today. There are so many sets and there are a few I'd like, but I tend to leave it for Xmas suggestions. I've not looked into second hand kits, but I did see some at a market a while back. They can be expensive.
I hope your sales go well. The postage costs can be high though.
!BEER
Yeah, some sets can be ridiculous money, especially if they're retired sets that are hard to come by. One vehicle I have is worth a fair bit, but I'm thinking of keeping that one, as it is fairly rare on top of the cost.
Some second-hand kits seem to be cheap, but sometimes come disassembled, which would be a major pain to sort, and construct, but could be worth it. Even if they're missing pieces, you can actually get the manuals online, and order particular pieces that are needed.