can the power booster be used with just 1 or 2 engines or is it for a much bigger layout with several locos
The power booster is not necessary for running 2-3 (maybe 4) locomotives, if you want to run say 5 locos or more at the same time then you can get the power booster at that time. Also, it's not too cheap either.
it went for 105 last night
Yeah, but if you would have bid it is likely the guy who would still have won or you would have paid around $150 if you won. The winning bidder's max bid was likely around $140