In your price range, I'd say you're probably better off looking on craigslist or e(vil)Bay. There are definitely deals to be had in the TB classifieds (I have done fairly well myself), but they tend to be on higher-end gear.
When I was in the market for a $300 bass amp, I ended up with a used Peavey Combo 115 (not to be confused with other Peavey 1x15 combo bass amp models such as Max 115, TNT 115, TKO 115, etc.). For the money I don't think you can do much better. However, this amp is HEAVY (~100 lbs.) and I ended up switching to a head and cabinet setup for portability as soon as I could afford it.
A guy I play with has a Fender Bassman 250 and I like the sound we get out of it. I think his is the 1x15 version; I'd have probably gone with the 2x10 instead, but that's just personal preference. If you could find a used one in your price range, that might be something else to consider.
The various Ampeg B-series 1x15 combos might also be worth checking out. I know someone who has one and it sounded pretty good. I think that one was a BA115HPT (i.e. 200W version with horn) and it might be hard to find a used one of those in your price range. However, for the regular BA115 (100W version without horn) you could buy used ones all day long for $300 or less.
For a new amp under $300, the only thing that really appeals to me is the Line 6 Studio 110. I'd have to say that's more than a little bit too small to gig with (although some people have purportedly pulled it off), though. With any of the other new amps in that price range, I'd be concerned about durability.