I get sell-on clauses from the selling club's viewpoint - free money

- but I'm astonished buying clubs accept them so easily. Having to shell out a big chunk of cash somewhere down the line doesn't strike me as sound business practice.
I don't get it from the player's / agent's viewpoint either. It must have a negative impact on a player's potential to move to a bigger club. Say Rochdale sell Mo Schmoe to Blackpool for £500,000, with a 30% sell-on clause. Bolton then want Mo and offer £1m. Blackpool need £1m to clear a debt, but they have to ask for £1.45m to get that £1m, because they'll be stung for £435,000 by Rochdale via the sell-on clause. The deal falls through. Mo's stuck at Blackpool.
Appearance / international / promotion / survival clauses are far worse. That's an almost completely random hit on the finances for a club that agrees to such conditions. It's idiotic.
I've said on here before that I followed Chelsea

when I was in London between 1982 & 1986. I was immediately roped into doing that. Very first weekend I was down there, Chelsea were playing Spurs in an FA Cup game (quarter-final

, last 16?). Two of my work colleagues were Chelsea fans and talked me into going. Chelsea were shite & skint at this time. This was a few months before Ken Bates bought them for £1

. They were halfway down Division 2. This was the Spurs of Ardiles, Villa, Hoddle, Hazard & Archibald. They were FA Cup holders. Chelsea nabbed an early lead, but were beaten 3-2. They'd a kid in goal - Steve Francis. The reason they'd a kid in goal was because they'd signed a decent back-up keeper called Bob Iles from a Non-League club, Weymouth, a couple of years earlier, I'd guess when Peter Bonetti was still first choice. When Bonetti "retired" to Scotland and played a few games for the Arabs, Iles's run in the first team was halted by a clause in his transfer deal. Chelsea had to pay a derisory sum to Weymouth (I mind it being £1,000, but I may be wrong) if Iles made 15 (

?) League appearances. Chelsea didn't have £1,000, so Iles, who was looking good, was permanently dropped after 14 games

and transferred out soon after.
Football's finances are almost always lunacy, but I don't get "sell-on" clauses at all.