As my Gretsch Brooklyn acoustic kit was in university I felt it was time to purchase a kit I could practice on at home, so I searched for a few weeks wanting to find a reliable and good kit that was in my price range of £400. I decided I wanted all mesh heads as the feel is very similar to that of an acoustic. I also decided to go with Roland's V-Drum series due to their reputation and quality, which leads me on to the first tip.
1. Go with a well known reliable brand
There are many good reasons for going with a well known brand which is applicable to buying anything in truth. I would say the three major electronic drum brands to go with are Roland, Yamaha or Alesis. The first reason for going with brand is, in the case of fault, part missing or something is broken on your kit a major corporation will have entire departments dedicated to your issues and will get it sorted a lot quicker. Also this links in with the second point of online communities with dedicated help pages, youtube demonstrations and other useful help pages if you have queries on the kit, setting up or how to use anything. When it comes to the time of resale people will also then be looking for reliable well known brands, making it easier to sell along the line.
Don't panic buy. Spend time looking for a kit at a good price, and don't settle for something you feel so/so about. Write down exactly what you want and then make a point of finding it. There is always leeway on pricing when buying used items so offer the lowest you think reasonable and work from there. You will always be guaranteed a good price this way.
3. Visit and pick up in person (if possible). Know what you're looking for upon arrival
Pictures can be deceiving. Check the kit in full before agreeing to buy. You can always change your mind once you have seen the kit. Maybe a little obvious but when you're there in person you might be a little eager and overlook any faults you find when you get home. So be thorough in checking and take every manual, box and guide the seller has that came with the kit. Also you have every right to email and ask the seller for more pictures and more in depth shots of the kit if you are buying abroad or even if you are visiting.
Making sure you have enough space wherever you are setting your kit up may seem obvious, but the last thing you need is to underestimate the space needed, come home, and find you can't set it up properly. Although naturally an electric kit won't take as much 'floor space' with cymbal, snare and tom stands, it can be wide. I underestimated the width of the kit and when I picked it up I was surprised at the width, as its a mini rack pretty much. So if needs be ask the seller how wide and tall it is to work out the floor space needed before hand.
5. Setting up
One thing I have seen a lot of is electric kits set up poorly. This will not only lead to potential damage to the kit, but it may be detrimental to your playing style and may make it harder for you to re-adjust when you cross back and forth to an acoustic kit. Set the kit up exactly as you would if you were doing so with your acoustic. keep the toms, cymbals and snare in the same position as your acoustic. This has been really helpful for me. Obviously in most cases the electric pads will be smaller but position wise this will help when returning to the acoustic.
Here is a list of sites I recommend using.
Facebook Drummers Buy/Sell UK Page
Ebay (make sure the seller is 100% reliable)
'Song of the week'
'Seafret - Atlantis'