If you are aiming for a profit you need to look at the type of things that are selling.
Go to search and type in the type of item. Then select sold items and scroll down. This will give you a better idea of what you should be on the look out for when buying things to sell.
Remember, eBay will charge you a business seller rate and also take a % in commission, postage is included in this calculation too.
PayPal I allow a generous 5% to be safe.
You need to factor any petrol and parking costs if you are not within walking distance of post office or courier drop off place. You need to subtract the cost of the product you have purchased and any pretty wrapping too.
It's quite a shocker. You need to charge much more than you think to break even.
Say you listed 100 items a month, divide your monthly premium for business selling by 100 and deduct this from profit.
Decide your price and postage and deduct 15%. This accounts for eBay fees, PayPal fees.
Calculate your journey time if In a car and work out actual costs including wear and tear on the vehicle and any parking, divide by the average amount of parcels you post in a single trip, and apply the portion to your costs for this item. Dividing up the cost of a bus journey is more straightforward.
The reality is that if you have a sharp eye you can make a slim profit sometimes a decent profit. If you don't you'll definitely make a loss.
My advice is stick to the market you know best.