Did being a benevolent slaveholder make it possible to take someone out of the slave market without power?
No. The context explains that in the 1840s and 50s, buying a slave out of the slave market in Virginia required money, wealth, clout, and influence. Although some slaveholders were benevolent and others were not, it still took a certain amount of power to remove someone from that market.
Les explains that in order to buy a slave out of the slave market in Virginia in the 1840s and 50s, a person had to have money, probably wealth, and some clout or influence. He acknowledges there were no doubt benevolent slaveholders, and some who were anything but, but he concludes that it still took a certain amount of power to take somebody out of that slave market. He uses this earthly picture to illustrate the spiritual realm: God accomplished redemption with His power, buying people out of the slave market, setting them free, cleaning them up, giving them a new wardrobe, a beautiful place to live, hope, and all they had to do was believe it.