Since I already have one (stripped) cabinet, I went to a local auction to see what else I could find on a budget.
I thought I'd share my adventures so others new to this kind of thing won't repeat my mistakes, perhaps.
Either way, here goes. Warning: rambles.
I set some basic rules before I went: Budget, usefulness, and what I needed/wanted. Then I bent and sometimes broke them.
Of course, there was too much visual stimuli and I managed to emerge with most of my money despite my actions.
What I did wrong: Impulse buy of a cheap, somewhat functioning pinball machine. Just because I don't have parts, and it would take $$$ to get parts for it. No shoving a jamma harness into a pinball case. Only saved by the re-sell right off the bat with a profit that took care of the other things I bought. Don't count on that kind of luck, though.
These auctioneers will gladly sell you something and hold you to it, even if it's overpriced and a mistake. It's a matter of seconds between losing a bid versus backing out when it gets too rich. These guys want to move on to the next item, so stay sane.
Some of these bidders have a working budget in the thousands, buying these things to make money, not a hobby,
so your desparate attempt to buy x game for less than $100 is nothing to them if they just have to put in one of their spare PCB's in the cab and resell it for double next auction or put in on site and rake in quarters.
Did I mention keep in budget? Get what you need?
Avoid the 'brand names' unless you really, really want that Galaga/Pac man etc. If anything, find a converted Midway; with the crazy prices going for intact ones, you can do a project one for less money and have pride of restoration.
Research *here* well before arriving. That kept me from being interested in, say, a Pole Position
because of the many systems that fail.
Things I didn't buy that I 'needed'. Centepede glass. Yep, I could have solved my marquee/bezel problems right away with a $75 purchase. The bidding went to $50 and I couldn't justify it, becuase, my cab isn't functioning yet, that money should be going towards those important bits that make the game work.
Of course, most items for sale there are full systems. That's where I went bargain basement. I wanted a cocktail cab, and got one for the right price since it's a funky conversion that I can unconvert and have something that fits in the living room better than the full-size cabs. . despite that I end up getting a second upright.
Since I'm pretty darn n00b on all this, I'm on a budget and have to pick from the stuff that these guys consider worthless.
Get there early, so you can tell what's a good game in the category of what you want, versus things like a Donkey Kong that pretty much was a box with parts related to the game inside, needless to say not working. Despite that, it went for $70.
I won four bids, resold one before I even left the parking lot, and just about came out ahead. Pure luck.
Now the rationalization: why I did what I did.
Box of joysticks (some new): Cheap. I still ended up spending less than one new controller, and I have some stuff I can either sell or install in the cocktail.
Cocktail: Well, if you can get a intact cab for less than materials to build one, so I went for it. Same price as the box of joysticks.
Pinball: A reasonable price. not 100%, working, though.. Why it was a mistake: I don't have pinball parts. The instant resell offer saved me. Who knows, I may have made a mistake in *selling it*, but I don't have the means or the contacts to do anything with it. Again, luck. Or the guy knew what I had and figured it was worth the trouble to make an offer.
Jamma cabinet: power supply, working crt, cabinet (duh), and jamma interface. Nice and generic so I'm not erasing any history if I decide I want to shove a PC into it. Even if the controls are bad, whoops didn't I just get a box of a dozen joysticks?
The reason I wanted a second upright is that it's a horizontal monitor, where my 'home' game is vertical.
Let's just say I stayed well within my budget, got two fixer-uppers and parts. But that was a hard ponder whether to keep the pin. Maybe next time

All said and done (finally, you say) I ended up spending half of what my local operators would sell me one of their generic cabs directly from their business. So I came out ahead, of sorts.
But it can be a dangerous place, so keep your head.