I personally would prefer to buy trees in stores rather than online as the delivery cost for a big and mature tree is just too much for the supplier or you to absorb. I have bought tree online, in stores and have found out that those online are bound to be shorter, smaller. You have to factor in the weight of the compost and the height of the trees.I bought some of my trees bare root when they are reduced in diy gcs. They are about 6 foot tall and yet reduced to £3, half of £6 initially asked for. I bought 7 of them, some peaches, some apples, plums, cherries and pears. When it is cold, you really can buy them bare root and grafted. Or opt for some reputable companies and choose dwarf root stocks. That way, even though your trees mature, they won't grow to enormous size or too big to handle. If you want to buy trees such as privet to grow as hedge, there probably is nothing wrong with buying them online. Make sure you have recourse if the trees arrive stressed or the suppliers haven't taken good care of them.