Alvey, Ebay takes a lot of time and patience, especially now that buying from the Americans has become so bloody expensive.  But it's surprising just how much good stuff has surfaced over here, like that Corbin Madras jacket I bought from a chap in Manchester just the other evening. 
Before buying, I check as many details as I can, including their feedback score.  Some people who leave negative feedback are simply chancers or imbeciles.  Good sellers take six or more photographs and are honest about condition.  I've been buying on Ebay and and off since around 2006 (at a rough guess) and have had about an eight out of ten success rate.  Where I wasn't happy, someone else on 'Talk Ivy' often was.  I passed a fair amount of stuff on, including an ancient J.Press sack to TRS. 
'Vintage' shops are often not worth the effort, but I would make an exception for 'Wild' in Nottingham, now owned by Shaun Hoolan, a former Russell Street shopper.  Not Ivy - more Americana - but with some nice - and inexpensive - bits and pieces.  Universal Works, run by the delightful Frankie (a young lady) is just across the street.

Just mulling this over whilst having breakfast.
I've been buying secondhand/vintage for roughly forty five years.  We young punk rockers took pride in cobbling something out of nothing.  Anyone who caught the train to London and spent money with Malcolm McLaren tended to be laughed at as a 'poser'. 
Later, the majority of stuff simply couldn't be bought new, at least not locally, often not at all in the UK. 
Russell Street was good for plenty, but by no means all, by the time I discovered it.  And names that kept cropping up on here could only be obtained secondhand, like the deadstock shirts (Lion Of Troy, for instance) I used to buy from a lady in Scranton.

Good pointers there from AFS. The main thing is to know your measurements from existing garments that fit you, insist on all the measurements you needl from the buyer. Jackets are a fairly good bet, but careful you don’t just end up with the top half of a suit! Tweed is so hardwearing it is often a good used buy. Trousers can be problematical as they get a harder life, plus I’m not struck on the idea that someone else has been farting or having sweaty bollocks in them.

Outerwear can be a good buy as overcoats and raincoats don’t seem to fetch a good price (nobody wears overcoats or raincoats now). But make sure you get the sizing right, chest size normally 4inches  bigger than jacket size - but measurements, measurements, measurements.

You also get to know that certain things will fit you; a 16R or 16-33/34 Brooks Makers shirt will invariably fit me well, but their collars can shrink with multiple washings so if I’m going to be wearing it with a tie I might go to 16.5. I’ve found that J Press shirts need to be half a size larger. Check collars and cuffs on shirts as they are the obvious places for wear.  40R Broooks suits and jackets are good for me but other makers sometime size up smaller.

Look out for ‘deadstock’ or ‘NOS’ (new old stock) in descriptions as this means its unworn.

Wool items need to be washed, dry cleaned, go in the freezer for 48 hours before introduction to your wardrobe in case of the dreaded moths.

Very much trial and error,  knowledge sometimes obtained the hard way. You may have to kiss some frog mouth pockets beofre you find your vintage prince.

^ Spot on and LOL at the sweaty bollocks bit.  I know exactly what Woof means.  Also, I bought a Baracuta raincoat just before Christmas with a funny smell to it.  I put it on a hanger and opened a window.  It improved.  Most sellers, though, wash and press. 
Go to 'Wild', you'll likely see Shaun in the back, steam-pressing.  A good and careful dresser himself, he knows what's what.
Bargains are to be had for the asking if the right approach is taken.
Funny, my new Corbin jacket looks not unlike the one the cartoon Woof is sporting ^.

Yuca might be a good source of information here.  He's both a purist and a precisionist.  Doubtless the two go together.

The other thing to remember about Ebay, of course, is that it provides quite absorbing online entertainment.  I can spend a good deal of valuable time looking at glass, DC comic-books, assorted Americana (aside from comic-books), Corgi toys, plastic soldiers, books and yet more books, postcards of 1930s actresses etc. etc.  I well remember, back in 2006, reading 'Superman's Friend Jimmy Olsen' before going off to school to invigilate an A-Level exam.
Alvey mentioned Nick Fury a couple of weeks ago.  I remember him well, although my Marvel heroes were Prince Namor The Submariner, Thor and - most of all - The Incredible Hulk.  It wasn't just that, though, it was all the great stuff on sale that you just couldn't buy in England: X-Ray Spex being at the top of the list.  Oh, what it is to be a baby-boomer!

^ Absolutely right in every respect.

Point 4 is of especial importance - the Watch List.  It allows you to cool off and you may end up getting the item (if you decide you want to bother) at a much lower price. 
Bidding is often like playing a fruit machine - but there are times when you regret winning the 'jackpot'. 
Last evening, I was offered quite a nice Brooks Brothers overcoat (made in the USA) for a mere £15.  But I really need to get rid of an overcoat, not acquire another.  So I'll pass. 
The same applies to the odd shop I might venture into.  You can end up buying something you don't really want just to show willing.

Something else to bear in mind - and I mean this quite sincerely - is, it's fatal to spend too much time on 'Talk Ivy' or anything similar.  It's especially fatal to go digging and delving into threads that were on here years ago, when we had more people posting.  How often did I - do I still - read about some Boom Years name - let's say, for the sake of argument - Cricketeer, Duck Head or Eastland - and I begin pondering then searching. 
Perhaps, for me anyway, it's no bad thing that so much that remains desirable is in the USA.  I very much fancied an old Geoffrey Scott button-down last autumn.  An inexpensive shirt but, by the time the seller had added on his shipping charge (which wasn't unreasonable) plus the slice this infernal government of ours decides to thieve from us, the cost of the shirt was up from around £9 to £30-plus.  It feels like a piss-take. 
A pity that Newton Street is no longer with us.  Ditto Dennis, the Highly Interesting Chicago plumber.  Farrago, too, was not averse to sending goodies across from Oregon.