I always assumed, though could be wrong, that clubs like Brighton, Brentford etc. started off buying players for lower fee's, selling those for good money, using that money to buy more expensive players than the first lot cost, sell those, reinvest, rinse, repeat.
So like, you'd buy a player for £2m, sell for £10m, then buy a player for say £5m out of the profit, sell for £20m etc. etc.
(they are example figures that i've just made up)
Rovers just wanna buy for £500k, sell for a profit and then buy more players for £500k.