This is about as close to a textbook example of "hostile work environment" sexual harassment in an office that you can get. Don't even bother talking to him again, just go straight to HR and if he doesn't stop immediately, get a lawyer.

By "party".....does he mean your feet, his penis and some lotion? That's pretty sick!

Instead of 'politely declining', be stern. Tell him no, tell him his repeatedly asking is wildly inappropriate, and tell him that continued requests will be reported to HR as sexual harassment. If possible and legal in your place of residence, get audio/video so you have proof that he's been asking and what your response was.

OP, please set your foot down and get to HR. Clearly this man has overstepped his boundaries with you.

No, he hasn't seen my feet. I found out he actually has asked nearly every person in the store because he hosts private foot fetish parties for clients. Part of me actually pities the man because he's just trying to make extra money, but I would never ever consider being part of those parties.

You need to report him

Can you get paid?

This is the point to stop being polite about it, I'm afraid. Sometimes being polite is taken to mean 'I will eventually come around to your way of thinking.'

Seeing as how this is involving a sexual fetish you should totally report him to HR for harassment. No one should have to put up with that behavior in the workplace. Tell that creep to crawl back under the floorboards!

This will continue until you take an active step to stop it or he finds a newer toy. Maybe you should make it clear that if he continues to pursue this line of questioning that you'll take it up with the boss and have his ass fired.