Ive been selling my old kit and i made sure it was all clean before sending. and i know people find it hard but i dont know why its simple just put in in the bath thats all i did. the only issue i have is cleaning gogs.
Break them down, give the lens a wipe down with a damp cloth and soak the other parts in mild antiseptic/disinfectant solution, wash them off with the shower head let dry and then reassemble it all. Straps, if paint stained can go through the machine on a cool/gentle wash in a mesh bag for protection (treat old/stubborn stains with washing up liquid). if you want them to look extra bling give them a quick spritz with some silicon dressing an a polish with a soft cloth, don't forget to wash the goggle bag etc.
HTH
I agree that receiving dirty kit feels like an insult, there is nothing worse than receiving a clean looking item just to discover that all it has had is a quick wipe over and as soon as you pick it up it oozes old paint.
Although I have Nicky Cuba's barrel sock that I have never washed
Obviously, if the buyer has a cyber crush on the seller then it might be a different situation