A sandbagger in general is someone who engages in sandbagging, i.e., deliberately underperforming in order to lull the opposition into false sense of security and strike at the right moment. A sandbagger in TE, in my mind, is someone who is L15 and playing in a L6 league. Like the guy who won my last league. Most of his first team were fully developed 5-6* with multiple positions and special abilities. He must have tanked several seasons to train them. I don't have a record of his team to figure out how he got into a L6 league. Perhaps he had a bunch of 1* players. This goes against the explanation that was given above, so I have to ask/think again how he did it. His average Q was not much higher than the rest of the league, but if each player has a special ability, that's worth about 3* by my calculation that is not part of the official rating. So perhaps the leagues are formed by average quality and not level after all?
The trick with selling off players at the end for a better cup draw does not seem very attractive to me. I would rather play and think like a real manager and build my team steadily by buying and developing the best players and being loyal to the best performing players even if their rank does not reflect their production. Around L5 and early in L6 I thought the rank was everything, so I sold off my start performers from previous seasons just because players with 4 more stars became available as my level increased. A lot of these acquisitions did not pan out. I have since read that newly acquired players often don't do well for a while. I don't know if that's the "official" community wisdom or just one manager's opinion.
Anyway, I won the cup the very first season I joined (where I was given a random team in the middle of a season and a cup draw). I won three rounds and the cup and thought "wow, that was easy". So at least I got that out of my system. I have won one CL and three leagues since -- before I came to the forums. The forums made everything harder because now I overthink everything. But I don't think I could have gone on without the forums. I was sinking in my L6 league fast and ended up clawing my way into the 4th spot in the last two rounds -- thanks to what I learned about the formations and tactics.