First and foremost: make sure you have a space in your house or apartment for the cabinet. No space, kinda dumb to look . . . hehe, well some people don't realize how big they are . . .
Second: buy locally so that you can rent or borrow a truck to pick up the cabinet and DRIVE it to your house. Don't buy out of your area, shipping will add quite a lot of $$ onto your purchase. FORGET ebay auctions or out of area sales!!!
Third: get an idea of what you are willing to pay for a cabinet. Prices I've heard range from $200 to $800 for a cabinet. I've been considering getting one for a while; I had a chance to get a 4-slot in excellent condition for $750. I passed on it thinking I could do better later.
Fourth: BE PATIENT, DO NOT BUY THE FIRST ONE YOU SEE OR ARE OFFERED. Wait until you find one you REALLY like (I want a 4-slot), and THEN start to act seriously.
Fifth: BEFORE you make an offer, check the entire cabinet out, inside and out. (You should already own a bunch of MVS carts to test in the machine). Test a number of different carts in it, and make sure everything works OK (speakers, lights, locks, etc.). Also, if there are features on the MVS board such as joystick connectors or stereo connectors, test them as well.
Sixth: Look for arcade operators and distributors in your area. Once you have your rough target price in mind, call them or, if you see a machine at an arcade or restaurant, ask them about it (or get the info from the machine, all operators label them with contact info).
Seventh: Don't accept the first price offered, always offer LESS. Negotiating is fine, if they get pissed about it then thats their problem. You can just thank them for their time and say you'll look elsewhere.
Well, that should all be a good start at least . . .