Re: marshall mg100 half stack
that's bad advice
play tons of amps, every amp ever, anything you can get your hands on, try them with pedals, without them, every amp responds differently to pedals, and has different sounding gain
then pick out the one you like the most, and can afford via ebay or other used recourse
THEN look for it used
remember to try the amps at the volume you intend to use them. IMO if you're gigging, you try them at gigging volumes, they're more important than gigging ones, believe me, i learned that the hard way
let the store owners know you're serious about buying an amp, and tell them you need to try them at gigging volumes before you make a descision. I'd leave out the part where you intend to ebay it though. However, if you have $600, and the amp is $600, then buy it from the shop, it's a courtesy thing.
The fastest way for them to get you to buy an amp or leave, atleast with me, is for them to leave you alone, and let you do your thing