Buying at auction is different then craigslist, I personally don't love conversion cabs but where you live determines price. We have a auction about 30 mins from my house 1000 pieces plus on average comes about 5 times a year, obviously easier to find one. I personally think that cab looks rough and would definitely try to beat him down on price a bit. See what he would take without the extra board and carts? I mean do you need two? Those two carts can be had for about 15 bucks each so not much value with those, maybe he thinks they are worth more and will knock off money for you? Really check out the monitor too and see if any burn or other probs.
Last neo I bought was an original two slot, good shape original cab. Came with a nice metal slug x and captain tomaday. The RED gun in the tube was shot but was able to pick up a 25 inch donor at Salvation Army and dropped right in. I paid 225 for that cab, but understood what was wrong with it, chances are if he had out a better monitor in it he would have gotten 300ish. Next auction had a 8.5/10 6 slot loaded with carts. Went for 600 but it was a small bid war, but at every auction neos looking like that seriously hover about 125 working.
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Conversions are cheaper and from the looks of that cab you may end up sinking more money into it cleaning it up so yeah, 350 seems high, but if you have no other options then you really don't have a choice :-)