The F50/1.7 is a good lens.
In my experience, you have about a 1 in 3 chance of getting an unsatisfactory lens on eBay. On the forum Marketplace here, you have less than a 1 in 10 chance of getting one you don't like - and even if you do, the seller is nearly always cooperative in taking it back.
Here are example photos from both the F50/1.7 and the DA50/1.8. You can decide if one suits you better:
http://pentaxphotogallery.com/photos/gallery/query?camera=&lens=810 http://pentaxphotogallery.com/photos/gallery/query?camera=&lens=4080