Welcome! The marketplace here is probably the best place to buy second hand lenses anywhere on the internet. I've bought most of my lenses here and have had nothing but good experiences. There is also good information all over this forum besides the marketplace so be sure to explore as much as you can. Enjoy!
Good to have you here, and I'm sure you'll enjoy shooting with your K-7.
The marketplace is excellent for what you're after, so all the best in your shopping - and as jolepp has in his signature, beware of LBA (lens buying addiction).