I often buy gardening items online. It's convenient and reliable, and although I often visit garden centres searching for plants and tools, I'm frequently disappointed by their narrow range of stock, leading me to turn online. Recently, though, I've had a few bad experiences shopping online that have changed my mind.
I bought 30 railway sleepers from a well-known online timber supplier, and paid in advance. I based my entire design for the Kitchen Garden around their advertised length of 2.6m. However, every sleeper that the company delivered was 2.5m, not 2.6m long. After much wrangling and chasing, they eventually exchanged some of them for sleepers that were... once again... 2.5m long.
I also sourced some mulberries and apricot trees, which can be difficult to find, from what was meant to be a reputable specialist. When they arrived I was shocked to see how badly the trees had been packed. Many of the main stems were twisted and damaged. This particular nursery guarantees their plants "until the spring." However, I'll only see how the trees fare over the course of the summer and autumn, so the supposed "guarantee" is useless. My follow-up questions to them about how best to prune them were answered unintelligibly. Customer service is also not their strong-point.
Buy local or ask for recommendations
All in all, a warning. If you have time to hunt around and buy locally then that's a good bet. If you can't find what you want and do need to buy online, then choose a company recommended by gardening pals. I'm happy to let you know the names of the companies that I'd recommend - as well as those I'd never buy from again.