From DominionStrategy Wiki
"Virtual +Buy" is an informal term for cards that do not provide +Buy per se, but can serve a similar function to those that do in that they allow you to gain multiple cards per turn.
Cards that provide virtual +Buy include:
- Black Market lets you buy a card during your Action phase which is completely independent from the normal process of buying cards.
- Extra turn effects (for example Outpost, Possession, and Voyage) give you an extra turn which comes with a Buy like a normal turn.
Gainers are distinct from sources of +Buy, but some card-gaining effects generate multiple gains from a single Buy. While many of these effects are very restricted, often gaining a specific card only, some a rather flexible:
For a gallery of extra turn effects, see here.
 Other virtual +buy effects