Sometimes, silence speaks volumes.
Or, it can simply mean an offer is too low to bother with, especially if the guy has already received offers higher than that.
If you need a reply, try asking for one. "Please let me know what you think of my offer, as I would really like your game and I don't want to spend the money on something else, or miss out on another game, while I wonder about yours. Thank you!"
And as another poster commented, using incorrect words ("if your interested") can't help.