Yes wether GG kids are popular, thanks to TV apperances - just make sure you price them well above what someone may be prepared for meat if that would be a problem for you.
But GG boys are not too big and usually fairly quiet, although we have not yet kept any past 11 months old, as by then the space in the shed was needed for new kids....
I have had good results with getting my goats sponged, in mid-October usually, the vet does t for me (I bring the goats to the surgery), and then the sponges come out 12 days later (I think, but maybe two weeks) and the goats are always in season two/three days later. I have only had two returning to service ever so far, and there were reasons for that, nothing to do with sponging or them not being in season.
I can recommend sponging, you can plan ahead as to when you can bring them to be mated, fore-warn the billy goat owner and agree with him/her what timetable would be suitable. But getting a billy rag is still necessary....