If I was that friend, I'd be confused, but If in need of money, then I would probably trade my used socks for cash (if those socks cost at least half less than you're giving). However, I don't think the conversation between the two would be the same for the next couple of days. I mean, it's weird, but fetish is always weird, so it's hard to tell. If I was you, I'd do it secretly without him knowing because you're not going to be sniffing the same pair forever, you'll want to get some fresh ones soon. But if one time would be fine, I'm not so sure about all the other times. Personally, I wouldn't be selling my socks to my friends twice a week, unless I wanted to make business out of it haha