The ては is short for 「~てはどう(ですか)?」, "How about doing ~~?", "Why don't you ~~?
~を買っては? = ~を買ってはどう(ですか)?
≒ ~を買ったら? = ~を買ったらどう(ですか)?
The ては in ~~てはどうですか literally means "If~~" (≒~たら).
(ては = the conjunctive particle て + the binding particle は).
So ~てはどうですか literally means "How is it, if you do~~?" --> "How about doing~~?"
It's definition #6 on goo辞書.
靴(を)買うならTシャツやいま使うもの(を)買っては?!
"If you're going to buy shoes, how about buying / why don't you buy a t-shirt / t-shirts or an item / items that you'll use now instead (rather than shoes)?"