Honestly, I don't see an issue with it. I read something a while back something to do with bottle brush bristles making marks and scratches in the lining of the plastic and bacteria can harbour in there. I'm honestly not sure exactly where I heard it, but that's what sterilizers are for IMO! I'm not too sure about the rubber valves needing replacing also, if you decide they would, then they're on the official MAM site for about £1 each. This isn't alot when you think about it, e.g. if there's 7 bottles, it will still potentially come up cheaper than buying brand new.
As for teats etc, I usually went up in sizes from Boots, as they sometimes have 3 for 2 offers on the teats. They're still not cheap, mind but I'm sure if you looked online you'd find some bargains on them. I've even seen them in original sealed packaging on ebay before, and if its sealed then surely it'd be fine?
MAM have a 96% acceptance rate (according to the packet) and we tried every bottle under the sun before LO stayed on MAM. I'd recommend MAM over other bottles e.g. Tommee Tippee etc as you can disassemble them. It meant any little crevices that can normally evade the bristles I was able to get into much easier. If the bottles are in good condition on ebay, honestly, each to their own. Sometimes saving money in one place means you can splash out in other areas, I'd say go for it! If it would really bother you, maybe open one pack of teats until you've tried baby on them and theyve took them, that way if you find you don't want them, you can always return or sell them? A close relative of mine also managed to both breastfeed and express, and her baby took the MAM better than any other she tried. xxx