I really do have to say caveat emptor. You listed everything you needed to list before you put up the phone it seems, sold it "as is" and if it doesn't work in Belgium or needs a sim card or anything else, that isn't your responsibility. You really have to do your homework before buying a used iPhone. So many things to consider and it is the buyer's job to ask all the necessary questions before bidding. I have been there before and please know that I am aware of where the line lies.
Things buyers have to ask before buying or else risk getting stuck with a lemon in their corner of the world: what is the frequency the phone operates on, what version of the firmware is it running, is it jailbroken and was the SHSH file backed up before jailbreaking, what was used to jailbreak the phone, what accessories come with the phone. Those are just a few. Then there is the question of whether the phone can be unlocked after it's jailbroken. Whew! I have not seen all of those questions answered in the average e-Bay sale ad by the seller and I frequently see "Ask questions about this phone BEFORE you bid on it, all sales are FINAL and the phone is sold as is."
I think you are being a super nice guy.