Relationships, continue, but can change. Remaining in touch with an ex, is nothing but positive. If current partner cannot see that, it is their problem.<br />
It would depend. If it was from my last ex I would question his intentions. I have a pretty decent friendship with the rest of my exes so I wouldn't be too surprised.
Ebeneezer tried to do me out of his share of the money he promised to pay me for last year's Christmas presents, so I find this question highly amusing.
If it was a freshly broken up couple and there were some unresolved feelings I would use that time to resolve anything and accept the gift. If it was an ex who it has been a long time and we both had committed relationships I would say thanks but no thanks unless we were all friends.
I would probably accept it as not to be rude. I would explain that I'm sorry but I don't have anything for him. I would not want to start a cycle of gift giving, that would be inappropriate and uncomfortable. It would never happen anyway so I have nothing to worry about :-)
I would give them thanks and invite them in for a drink.