I would be interested to know where the slaves in your poster were being sold. There were no legal slaves in England or Wales since Medieval times.

Slavery was not legal in England. It was found to be so in a number of famous court cases.

By "not legal," it was not legal for a man to be a slave in England. If a slave landed in England he or she became free. It was not illegal to own slaves (though they would become free if brought to England).

The first court case was "Smith v Gould" in 1701. Sir John Holt, Lord Chief Justice of England ruled, "No man can have property in another. There is no such thing as a slave by the law of England."

In 1705 in "Smith v Brown and Cooper", Chief Justice Holt re-iterated the same ruling, "As soon as a Negro comes into England he becomes free. One may be a villein in England, but not a slave".

Obviously, there were still some slave owners who brought their slaves into England and tried to get away with it, because there were several other famous court cases: In 1762, in "Shanley v Harvey", Lord Henley ruled, "As soon as a man sets foot on English ground, he is free." In 1772 "Somersett's Case" again held that the condition of slavery did not exist under English law, and that slaves who arrived in England or Wales were free.

Slavery obviously was legal in many parts of the British Colonies. Britain banned the slave trade in 1807, and abolished slavery throughout most of the British Empire in 1833.

Source: http://www.anti-slaverysociety.addr.com/huk-slavery.htm

Obviously, some slave owners tried to bring slaves into Britain (or there wouldn't have been so many court cases, brought by slave owners trying to get their run away slaves back, or by slaves trying to establish their freedom).

Some slave owners presumably got away with it. The slave would need to know about their rights, or, more probably, they would need to encounter anti-slavery people or groups who would inform them of their rights and help them.

In any event, there were very very few slaves in England, and any who were there were not legally slaves (though they might not have known this).

For all these reasons, I would be very surprised if anyone was selling slaves IN ENGLAND in 1820. They could very well have been selling slaves in English colonies, as slavery was not abolished in the Empire until 1833.