A few years ago I used to supplement my income by selling books on Amazon. Although I no longer do this I thought I would offer a few tips in case anyone else is interested. Some are also probably relevant to general online trading. I am now a poacher turned game keeper and help at a local charity shop and sometimes spot items they can sell on ebay.
Most people start by selling their old books, often college textbooks but then need to look for replacement stock. The first thing I realised was that online selling is more about the ability to source quality stock cheaply than brilliant salesmanship. When you first look at Amazon you will find a lot of books for sale for 1p and you wonder how sellers manage this. The answer is that these are mega sellers who probably buy charity shop rejects for about 5p each and make most of their profit from the postage charge. Professional sellers also pay considerably less Amazon fees than you or I.
The key to success is to find your niche. If you are interested in fishing or local history that is the place to start. Have a look at sites like Amazon or Ebay to find what is selling and for how much. If you try looking up the price of your own books it will give you some idea. All modern books have an ISBN number above the barcode if you search for this number you will be able to compare your edition with what is on offer. Try to do a practice listing to see exactly what you will receive and then decide if it is worth it. Also look at the best seller ranking to see if it is likely to sell. Personally I would avoid hardback fiction or thick mass market paperbacks. Old encyclopedias , readers digests or family bibles are also unlikely to sell.
The next task is to try to source the books in good condition cheaply. If you have time go into your local charity shops and talk to the staff, ask them to look out for books on your interest and if they find any pay a fair price and thank them. Large book shops also have end of season sales especially if a new edition of a text book has just been published. Unless you intend to trade on a large scale and have a lot of storage space I would avoid buying in bulk. It takes a surprisingly long time to list and pack books. Only buy books for resale in very good condition.
You then need to cut costs for posting and packing. I was able to buy padded envelopes in bulk from our local stationers. Look on ebay for discounted stamps. It is a good idea to invest in some cheap postal scales. It is important to work out a selling price that gives you a reasonable profit. Some dealers have software that automatically makes their books a 1p cheaper than the next lowest but ignore this. If your books have been well chosen they will sell. Give the books a good description, mention any faults like highlighting in textbooks and try to package them well and post promptly. If you have good feedback you will get orders.
The Amazon sellers forum is very helpful. If you have several books on the same subject or by the same author it might be worth selling them as a bulk lot on ebay. More specialist books sometimes sell better on Abe books which is also owned by Amazon. Ebay also allows you to let the buyer collect if they are particularly heavy. Ladybird books have devoted collectors and prices can be checked on the wee web or they can be saved and sold as a collectors lot. Another series to look out for is Haynes car manuals. I once sold a manual for a reliant robin.
If you have books that are worth a few pounds but you just want them out of the house a site like Music Magpie is worth considering. Minimum sale is £5.00 but they pay postage and provide the packing labels.