Why was he not purchased before Virginia?

When Olaudah's ship arrives in Barbados, he's taken ashore with all the other slaves. He gives a very detailed description of the process by which the slaves are bought and sold at the market place. It is a particularly harrowing experience for Olaudah; he sees friends and relations forcibly separated from each other, never to be together again. The slaves are not regarded as human beings; they are simply items of property to be bundled together in convenient lots for sale, even if it means keeping family members apart.
Olaudah, however, along with a number of other slaves, is not sold at the slave market. He mentions earlier that the slaves were terrified when the buyers rushed into the yard to make their purchases. Olaudah must also have been visibly agitated at what was happening. He appears to suggest that his extremely anxious appearance was the reason for his not being sold:

[W]hen I and some few more slaves, that were not saleable amongst the rest, from very much fretting, were shipped off in a sloop for North America.

"Fretting" in this context would appear to be mean anxious or extremely worried. That's not to suggest that the buyers at the slave market took pity on Olaudah and a handful of other slaves because they could see worry written in their faces; it simply means that they thought such slaves might be difficult to handle and control given their highly agitated state. The buyers might have concluded, then, that making those additional purchases would not have been worth the trouble.