The SE535 is certainly something to look for. Used is a great route to get good products on the cheap. I wouldn't expect much depreciation on the SE535 yet though. However, you might look for some SE530 earphones instead as their value is going down. They never really seemed to hold high resale value anyways versus their new pricing. The Triple.Fi 10 might be one of those too. Some of these earphones have that fear of breaking or wearing out in some way. The Triple.Fi 10 has cracking and the memory wire cord section. The Shure SE530 had the wire hardening and cracking. People tend to fear these a little more than they should and resale value is hurt(good for you).
$300 is plenty to work with on the used market. It' lets you buy most anything out there.
In terms of raw SQ, I see the Audio Technica CK10 as one of the best options out there, if not the best. It does so much right. The only problem is that it has a narrow but sizable frequency response peak at 10kHz that makes the treble a little hot and spitty. EQing tames it down, but stepping to a Comply T-500 tip does too. The open cell foam tip soaks up a bit of the highs and tames down the top end to an acceptable level.
I personally prefer the HifiMan RE252 over the CK10 as it is more balanced with no work and is a little more direct in presentation. I have come to view this as a reference level product, extremely good, highly revealing, and scales with the quality of the music. The CK10 does offer more micro detail and has a more real sound, but the RE252 is more balanced across the spectrum, offers more note impact and energy, and is a little more direct between you and the music.
Both of the above are well within your budget.
Really there's tons of great options out there and $300 can get you most everything. You really need to read through reviews and figure out which kind of sound you are shooting for. I've never used the SE310, so I'm not sure what kind of sound you're coming from. Most anything you will be looking at will probably be a pretty sizable upgrade anyways, so you will be happy with a lot of the higher end products out there. It's simply a matter of which one best fits your personal tastes.