Package everything as if you expect it to be delivered by a large catapult! (Double boxing, lots of shock absorbent 'filler' between boxes that won't shift in position during transit.)
Use ebay 'filters' to block bids from overseas (unless you welcome the extra hassle) and from people without PayPal accounts etc.
Take good, well focused and well lit, big photographs (and lots of them) showing the item from all angles (including the fact that you still have all the original manufacturer's inner and outer packaging, instructions, accessories, transit bolts, interconnects, mains leads etc.).
Ensure you send the item with recorded delivery (next day/next morning is best because it spends less time being processed) so you have proof the buyer received it.
Insure the item for at least what was paid. (I don't pass that bit on to the customer as that's my decision to cover for any refund if it is damaged/lost by the courier.)
Don't spend any time on time wasters. ("Can I come and listen to it with my amp first". No. You are not a dealer or a business and your responsibility is to ensure the item is exactly as described and not to act as 'showroom'/'demo-lounge' to all and sundry.) There are usually plenty of other people who know exactly what they want so you don't have to bother about the ones that don't.
Set a reserve so your £500 amp doesn't go for 99p by some fluke.
Be honest about the condition and ensure any defects are accurately (and clearly) explained and/or photographed in the description so there is no doubt.