What we are talking about is picking up more flying in a vacation month. Here's an example. You have a week of vacation, your PBS bid awards you 3 trips, plus the week of vacation, your line is worth 80 hours.
But now after bid awards, the vacation credit time (22:45) is taken out of your line, it's now showing only 57:15, so you can now pick up an extra trip worth at least as much as the vacation time was, since it is not credited towards your max pickup limit, only to build the line in the first PBS run. You white slip a 25 hour trip and your line is now worth 82:15, and your pay for the month will be the 82:15 PLUS the original vacation time of 22:45, or 105 hours for the month.
Rather than staying home and enjoying your week off, you pick up another trip, or two trips, one 2 day on each side of your vacation week.
That is 'selling back your vacation.'
For every 3 guys who do that every month, that's one less pilot needed to fly those trips the vacation guys picked up.
Yes, I know you have to fit another trip into the remaining days off, and that could be tough for a domestic guy, because we can't pick up anything in the week that was our vacation, but a few months ago there was a guy on here saying we should be allowed to pick up more flying during our vacation week as well!
We are our own worst enemy when it comes to flying too much, and then crying about slow upgrades!