Ebay may be shonky for some people after gpx5000's, but I have never had an issue on ebay, it's Gumtree that you should be steering clear from, only buy off them if the detector owner lives near you, and you can personally see and test the goods.
Nearly all my detector have come off ebay, some locally, some from the US. Never had an issue, an have always dealt with honest sellers with good feedback. There are some serious $$$ to be saved buying a second hand detetor from abroad, sometimes more than half what you would pay here, and that's including postage. So long as you aren't too fussed on warranty, or are willing to send the unit back to the US for repairs, or pay for it to be fixed locally from our dealerships. The only detector I wouldn't buy from overseas is a gpx5000.
I feel we are getting off topic suggesting expensive PI detectors, when the stated budget is only in the $200-300 range. If you want to get into gold detecting on a meagre budget (as not everyone has thousands to spend), there is nothing wrong with making use of some of the older Minelab, White, Fisher and Tesoro detectors out there. Whilst not in the same league as the latest detectors, many people still own and use them, sometimes quite successfully. The learning process on these detectors can be fun and very educational for a first time gold detectorist, and will hold them in good stead should they chose to upgrade in the future.
Detectors that come to mind include older Minelab detectors (FT16000, XT17000, Gold Striker), all have auto ground tracking, but the better picks are the 17000 and striker with the higher 32kHz frequency for small gold, whilst the ft16000 is more about depth than sensitivity to small gold.
Whites models include the Goldmaster 2 and V/SAT models, both excellent for small gold, running very high frequency at 50kHz which is hot for small gold, and similar frequency to the current model GMT. Both have manual ground balancing, with the V/SAT model having the edge with its adjustable vsat controls.
Older Tesoro Lobo models are still well regarded for finding gold, the australian released model was called the Dingo, but is pretty rare with its Australaia only changes to the ground balance capabilites for our hot ground. They also come with good discrimination and manual ground balance controls. A normal lobo should still be considered.
Fisher had the original Gold Bug on offer, it also had manual ground balancing, and was quite the detector in its day. You see some of these pop up on ebay and forums from time to time.
Garrett had the Scorpion Gold Stinger, which was quite a sensitive detector despite its lower frequency, but has a bit of a learning curve. It has some quite good features, but was let down by requiring 3 9v batteries to power it.
Basically don't bother with any of the cheapy chinese or cheap coin detectors, none have the ability to ground balance to make them usable on the typical hot goldfield soils, at least the above models do have that ability to a degree, so that's one step in the right direction. Accessory coils are still available for most of the above listed detectors, minelab apparently no longer service their older models, but I believe Tesoro, Whites, Fisher and Garrett will still do repairs (some may require shipping to the US).
Don't be afraid to look to the US for buying these, they are more common over there, and can be had quite cheaply. Many of these detectors when sold in Australia still command good prices, despite originating from the 80's-90's, as they are generally sought after by either people just starting out, as well as collectors. Other than that, ask around on the forum, some members may have older unused detectors they may want to part with.
Myself, I have a softspot for the old Whites Goldmasters, very sturdy and capable machines, plus you can still get gear for them (three different coils, shafts, battery packs etc).