Personally I prefer to buy new from places like H&M rather than buy higher-end stuff on eBay (then you can return it if it doesn't fit or you don't like it). I find H&M vest tops (scoop neck and camisole-type ones) to be excellent, and also have three pairs of their trousers (chinos and linen ones). I've also had a BRiLLIANT pair of grey skinny over the bump jeans from there, although they don't seem to have them any more, and very good over the bump leggings. A word of warning, though - all their pairs of leggings seem to be of wildly differing length, with no correlation to the label size, so it's worth trying these on instore if poss.
Other than that I've had some nice bits from Mamas and Papas in the sale (great shorts) and am wearing jersey dresses that I had pre-pregnancy, as well as some short-sleeved non-fitted tops that still fit me.
Someone pointed out to me when I first got pregnant that you may as well buy maternity clothes relatively early on in your pregnancy and get good wear out of them, as you're pretty certain to end up needing them for the last couple of months (and maybe after the birth) anyway, and I do think this is a good point.
Also, bras: I bought a couple of cheap underwired t-shirts bras at around 3/4months but am now feeling the need for proper maternity/nursing bras. Following a couple of recommendations I ordered a Bravado Silk Nursing bra (around £27) and it is super-comfy, although I'm swapping it for black as the "Latte" colour is a ridiculous colour that no-one's flesh actually is, so doesn't work as nude.
Hope this helps a bit!
P.S. It's worth flagging up that there are very few places that do mat wear instore - most of it is online - the only places I can think of that also have it instore are H&M (varying selections) and Topshop (fairly terrible selection, IME), and of course specialist places like M&P, Jojo Maman Bebe etc.